1.10: Relationships
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Expressions of and searches for love and sexuality pervade all corners of the web today, particularly the world of mobile applications. If there is one area of life that we can say has been deeply changed by technology today, it is the conflagration of love, sex, loneliness, and marketing which we will simply call online dating.
Sexting and selfies
As always, history reveals that the changes enacted through technology on online dating have been gradual; that there is a great deal of convergence between how we have loved and lusted in the past and how we do so now. For example, this video by the Atlantic Monthly provides examples of sexual texting or “sexting” style messages exchanged among lovers and lusters in generations and centuries past.
Selfies are a new genre of photography, and art, according to Jerry Saltz in this article for New York Magazine . They also may signify a new sense of self-reflection that is redefining romance and sexuality. For example, images of nude women have been circulated on every type of medium used in history. But in the age of selfies and sexting, women are now the most frequent photographers of the female form ; the sexualizing gaze is directed at the self, and then shared with the world. In her article entitled Sexting as Media Production , Amy Hasinoff promotes the idea that sexting can be empowering for young women. She also unveils the complications when laws designed to punish people for circulating pornographic images and abusing children are applied to young people who are expressing sexuality over phones and online. As seen in the erotic photos just above, the online circulation of sexualized images is an old practice with new players.
How social media leads to self-obsession
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My 500-Word Story on My Unique Perspective on Social Media
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Social media has always had a big impact on my life. Around the age of 11 I was invested in playing the games on Facebook. My mom got to the point where she was so tired of having to log into her Facebook for me to play the games that she allowed me to make my own social media page. This is what started the slope of my life into the social media world. My knowledge on social media is extensive. When I was entering high school Instagram had just become a very popular platform, and having started social media at 11, by 15 I had almost every social media there was. Though this was the age I became invested into making a good instagram. I followed , who posted their luxurious lives, and I strived to have mine resemble theirs.
This is when I began to develop a knowledge on editing photos. I downloaded an app called FaceTune where I learned the interworking’s of photo editing. I eventually got so good I could completely change the background of the photo and edit out anyone’s insecurities, even items in pictures. I also became invested in the Adobe editing world with an app called Lightroom. The Famous Influencers on Instagram I followed, were selling edit’s to photos that made your feed look a certain way. I was so obsessed with the concept of a color scheme, and all my photo’s looking the same on social media, that the photo’s I posted didn’t even resemble me.
 I had become so good a using social media, that I wasn’t being my self. I had looked more at how many followers I had gained then the positive things I was doing in life. Social Media can consume a person, once you gain even the slightest bit of knowledge. That is what happened to me. Even before going to college, I had become obsessed with how my outfits looked, how many people commented, and how many likes I got on every photo. I would use my Instagram algorithm to see who was interacting, what the best times to post were, and what type of posts got the most likes. This lead to me having ‘photo shoots’ to get just the right picture, because I had to look perfect on Instagram.
The obsession with Instagram, is what lead me to being insecure about my body and the way I looked. I eventually had to take a step back and see that my knowledge and exposure to social media is what was consuming me. I needed to stop letting my world be revolved around getting a perfect picture, because I wouldn’t enjoy and experience since I was too focused on taking pictures. I really wish that I hadn’t had the exposure to social media that I did because I think I would have been a lot happier with my life if it weren’t for social media’s effect. I hope as I grow older I can start living in the moment and using less social media.
About the Author
Savannah Williams is a Communication Major at the University of Arizona. She is from Scottsdale, Arizona. Williams one day wants to open up her own business in her hometown.
Connecting to the network and infiltrating the tribe
A significant part of human social life is organized around who we form lasting romantic relationships with. The online world, once idealized as a place of anonymity and separation from offline life, now has networks devoted to replicating offline life online, and building networks of relationships. These interconnected relationships are what experts really mean when we talk about social networks , which sociologists began studying decades before online social networking sites existed.
Dating on Social Media: Authenticity Vs. Structure
Student Content, Fall 2020
Boyfriends and Social Media
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Music: Spirit (Final) by Aritus, CC BY 3.0 .
What Social Media Means to Me
For me social media has always been a safe space for me to be my authentic self. Starting it in the 5th grade, I had no intent to show off or impress anyone. I would post maybe five times a day and I wouldn’t even care what I posted about. However, I noticed recently that I have started to fold into the mix of overthinking instagram and thinking too much about the traditions within it.
Instagram is so big that it has its own culture, its own language, its own traditions. And as users we have the option to adhere to these or follow our own path. The societal pressure, and especially being 19, it’s hard to not just follow the status quo. I have tried to go down my own path of not caring what other people think about my profile, but over time it has gotten harder to not just post for me. You find yourself wanting to post to impress your mom, friends, and other acquaintances.
In my podcast I discuss with my boyfriend Alec, the peer pressure I felt of us posting about each other for the first time. As a guy, who despite his resistance, follows his own set of traditions on social media. We relayed how as a culture we perform or act on our posts. As a little girl the biggest deal to me was to be able to post pictures of my boyfriend as I got older because that’s what I saw my older cousins do and what I was looking forward to be able to do. When Alec and I were able to visit each other during the summer, I knew that we had to get the perfect photo for instagram. And for the most part there were great moments of just us living in the moment, which I really tried to stick with, but then I remembered the pressure I felt. This pressure stems from years of social anxiety and having my ow insecurities.
Talking with Alec made me realize how much power I give social media, and I think that goes with a lot of other people. We are given these standards or traditions and told to stick with them, and if we don’t we will be criticized. And I think that nowadays there is too much power that social media gets, power I no longer want to give.  And as much as it was great, it wasn’t perfect, because nothing is!
Overall, what I learned from this experience and being able to state it out loud was that I no longer want social media to have power over me. I want to be able to have freedom and express whatever I want. Whether that’s a political opinion, an outfit I would have otherwise not shown, or really goofy photos that I would have normally been too embarrassed to share. For the sake of my mental health and overall enjoyment of the app, I want to let social media be fun and not always have to follow the typical traditions that come with it.
About the Author
Alyssa De Leon is a current sophomore at the University of Arizona. She is a film and television major from northern California. She spends quarantine watching reality tv and baking.
Dating is important. It offers the rare invitation for someone unconnected with one’s social network to enter, at least temporarily. Dating can be the preface to an intimate relationship and potentially a lasting one. So this invitation can’t be extended to just anyone; we have to vet people before they can enter the inner sanctums of our lives, and biases can play significant roles in that vetting. The discomfort of this process for a new partner of a different race is captured in two films about a young black man coming to the home of the family of his white girlfriend: 1967’s Guess Who’s Coming to Dinner , and 2017’s Get Out , which explored these themes through the lens of horror. Yet even when dating does not end in horrific consequences, it is still a high-stakes activity.
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But is dating always so serious? What if the connection is intended, by one or more of those involved, to be just about sex? Well, that is part of human history as well. And it still has significant emotional and social consequences.
The sequence for seeking someone special
In a 2014 article entitled From Facebook to Cell Calls , Yang and coauthors found that college students progressed through layers of electronic intimacy – different media chosen as benchmarks in the progression of a romantic relationship. When they were interested in someone, they began by connecting with a “crush” through Facebook, which allowed the “crush” to see who their admirer’s friends were and how the admirer looked, communicated, and behaved. The next layer was instant messaging – direct communication, but not as direct as the use of one’s “digits” or cellular connection. After instant messaging, they moved to the more intimate sanctum of text messaging. The final step was a face-to-face meeting. Overall this sequence of media they used followed a pattern: they began by performing in front of and viewing one another’s social networks, they then moved into more direct one-on-one communication before meeting in person.
Data in the above study was likely collected in 2011 or 2012. So what might people like the participants in Yang and coauthors’ study be doing to find romance now, five years later? College students today may be using some different platforms in their pursuit of new connections than the students in Yang et al’s study; Instagram is likely high on the list .
However, it is also likely that at least some of the pursuit of romantic interests that happened through different media in the past is now consolidating in online dating sites. The Pew Research Center published a report in 2016 about the growing number of Americans who have used online dating. They found that online dating usage by those aged 18-24 has nearly tripled since 2013 and usage by those aged 55-64 has doubled; other age groups’ use has increased as well.
Do dating apps do what we did before?
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Online dating apps afford the presentation of ourselves to prospective friends, partners, mates, and hookups. On these apps, users’ imagery and self-description tend to be idealized, sometimes to the point of deception . Apps such as settleforlove.com have been developed around the desire for more honesty in online dating, but their market share has not been spectacular. It seems upfront honesty is not the best way to gather a public of potential lovers.
Do dating apps also follow the sequence found in Yang and coauthors’ study, moving from social and tribal to direct connection? That depends. Some apps leave out learning about someone’s social connections, relying instead on complex algorithms to calculate who might be a good match – even if scientific evidence does not show that these algorithms work . Others just speed through the sequence by facilitating immediate direct connection , and in some cases, quick sex. Some use the language of sociality like “tribe” and some connect you to matches through your social networks .
But we humans and our cultural norms still determine a great deal of how dating apps are used. Just as bias affects algorithms across the web, bias has been found to tip the scales on dating sites in favor of white men, to the detriment of groups including black women and Asian men. Sites and apps follow our leads as much as we follow theirs. And apps only go so far; dating apps today function more like online shopping than like relationship formation of the past. In the BBC Horizon film How to Find Love Online , the romance-focused anthropologist Dr. Helen Fisher says they are better called “Introduction services,” with the act of dating and the final vetting before it still conducted in person.
Mobile dating apps
Mobile apps are particularly influential in the world of online dating today.
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And then there is Tinder , the most popular in the US (although not the world) at the time of this writing , which has taken the heterosexual dating world by storm. There are many critiques of Tinder’s effect on heterosexual dating, however, including studies finding that it favors men’s usage norms over women’s . Tinder faces strong competition from numerous competitors for the US market, however, including a direct challenge – with a grudge – from a Tinder cofounder’s site, Bumble , discussed more below.
Connecting, Tinder, and the Social Media Coma
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My Perspective on Social Media Use
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Living in this day and age, social media is used by millions and millions of people all around the globe. I enjoy using social media for purposes such as being social (obviously), keeping in touch with my friends and family, expressing myself, sharing some of my own personal thoughts, beliefs, and opinions, and having a platform for myself to share whatever I want, whenever I want.
Social media usage comes with many benefits and positives. However, there are also many negatives that come along with being on social media. In my own personal experience, I have gotten my feelings hurt over seeing people doing something without inviting me, leaving me to feel left out and excluded. Also, people tend to be very harsh and rude over social media because they are behind a screen and not face to face. Therefore, they may be more confident in the cruel words that they are typing. I try to use my social media as a positive additive into my life. I feel like there is already so much hate, judgement, and negativity in the world, so why should there be any more from being on social media (which is supposed to be fun)? Personally, I choose to follow accounts that preach positivity and lift my spirit, and then unfollow all the others that do the opposite.
Social media can also be very time consuming. Some days I have found myself buried in my phone on the couch or laying in bed for hours upon hours as the day wasted away.  It is moments like those that make me feel guilty and ashamed that I actually spent countless hours on my phone, examining other people’s lives, when I am actually wasting my own by doing so.
Another aspect of social media that I do not participate in, unlike many high school students, college students, and young adults is online dating/ hooking up apps. The most popular one among this generation is Tinder. Tinder is a “dating” app where you basically judge someone in a matter of a few seconds on their looks, age, name, location, and biography (if they have one). Then, you either swipe right or left, depending on if you like what you see or not. If you match with someone, meaning both people swipe right on each other, then you can arrange plans to start talking, hanging out, or whatever else. I have an old soul and would much rather find a life partner in a more natural way… say maybe bumping into someone at the grocery store and locking eyes and feeling immediate sparks (very cliche and seen in almost every rom-com, I know)! I don’t think that there is anything wrong or shameful about Tinder or other dating apps, but I just choose to not be a part of them.
Overall, social media is very prevalent in today’s society. It can provide countless opportunities and knowledge, just at the click of your fingers. I think that using social media in moderation is a good thing to do. People should also be uplifting, supportive, and kind to one another over the screen to help make social media a healthier, happier, and safer environment.
About the author
Marissa Farhi is a college student at the University of Arizona. She loves to work out, spend time with friends and family, and eat avocado toast.
The paradox of choice
Some scientists and users are critical of online dating apps because of the wide selection they provide. As Aziz Ansari points out in this podcast episode, and in this article , for some people dating once meant choosing from an extremely small selection of people. He and the podcast host cite studies finding a “ paradox of choice ,” psychologist Barry Schwartz’s theory that the more selection we have, the less likely we are to choose something and feel satisfied with our choice – whether it is a partner or a jar of jam. And today? Thanks to these apps, users get exposed to a lot more jam.
For users in big cities, it is possible to swipe almost infinitely through prospects for dating and potential sex. As my friend Mary Franklin Harvin describes it, it gives “ an air of disposability ” to people. Nancy Jo Sales’ Vanity Fair article on Tinder goes further, claiming women have fewer orgasms in the numbers-game exchanges Tinder facilitates – and in these situations with so little intimacy or rapport for feedback, men do not learn the skills to be good lovers.
Many in the article linked above, including Sales, have charged that Tinder encourages a culture of harassment of women. That may be in part because the culture within the company has been the site of harassment. Tinder co-founder Whitney Wolfe left Tinder in 2014 after being sexually harassed there, received a settlement, and started Bumble.
But online dating sites, like most technologies, depend on humans to use them and shape their norms. To end with a ray of hope for those who feel they have to use Tinder, artists like Audrey Jones , Matt Starr , and Jarrod Allen use Tinder to make art. If nothing else, they remind us that humans can choose to use platforms in new ways – even if using them differently than the crowd can be lonely.
Core Concepts
selfies
a 21st-century genre of popular art and media production
layers of electronic intimacy
a term by Yang et al (2013) to describe how college students chose different media platforms as benchmarks in the progression of a romantic relationship
Sexting as Media Production
a 2013 article by Amy Hasinoff promoting the idea that sexting can be empowering for young women, and unveiling the complications that arise when laws designed to punish people for circulating pornographic images and abusing children are applied to young people who are expressing sexuality over phones and online
paradox of choice
psychologist Barry Schwartz’s theory that the more selection we have, the less likely we are to choose something and feel satisfied with our choice
Core Questions
Choose the best answer:
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Question for Qualitative Thought:
Consider the layers of electronic intimacy discussed in this chapter, and then consider your own generation and subculture. What layers of electronic intimacy do people you know often go through as they try to get to know another person?
Related Content
4 tips for staying connected during coronavirus, from migrants who live far from family (by Lynette Arnold for The Conversation)
Lynnette Arnold , University of Massachusetts Amherst
As social distancing and shelter-in-place orders are implemented to curb the spread of coronavirus, ever more people worldwide are separated from relatives, friends and loved ones. As of March 29, an estimated 229 million Americans , 60 million Italians and 1.3 billion Indians have been asked to stay home.
Forced separation, while new to most, is a fact of life for the world’s migrants . Still, many sustain close relationships with relatives through years, even decades, of physical distance.
As a linguistic anthropologist interested in the power of everyday language , I study how such families maintain relationships by analyzing recordings of their conversations. I worked with migrant families living stretched between El Salvador and the United States to record 75 hours of these conversations over four months.
I’ve identified four communication strategies of long-distance families that may help people in coronavirus-related isolation cope with physical separation and maintain social connection.
1. Nothing beats a good phone call
As millions more people rely on video-conferencing technology, they’re discovering what migrant families have long known: Video communication can be draining – physically, mentally and emotionally.
For work-related video conferencing, the challenges include preparing yourself and your immediate surroundings for broadcast. For families, studies show, seeing those you love on video can make feelings of separation more intense , increasing the yearning to be together.
Text messages and other written communication, on the other hand, can feel too impersonal and may not be accessible to those – like young children – who can’t read, or to people with visual impairments.
That’s why the families in my research rely on phone calls for almost all of their communication. Calls are intimate. Hearing a loved one’s voice can convey emotions without the constant visual reminders of separation.
2. Communicate to connect
Communication isn’t just about conveying information – it’s also a way of caring for others . The long-distance families I study use small acts of communication to reinforce their intimate bonds.
Take greetings, for instance.
Such elaborated greetings articulate family relationships that stretch across space.
As migrant families speak these greetings over and over, in every conversation, they continually create and re-create connections despite distance. That’s because communication has consequences beyond the moment in which it happens. Research shows that migrant families are aware of how the effects of conversations accumulate over time.
3) Manage conflict carefully
These long-distance relatives have also developed strategies for communicating about conflict, which is inevitable in any family, and minimizing its consequences.
By and large, the conversations I analyzed avoid explicit disagreement and signal concerns in subtle ways.
For instance, if relatives in El Salvador don’t have enough money to cover their day-to-day costs, they embed indirect complaints as they recount family news. A story of their father’s visit to the doctor, for instance, will include a quote from him lamenting that he cannot afford his new prescription.
This communication strategy puts family problems on the table for discussion without placing blame.
4) Celebrate the past – and a future together
Communication has the ability to span not just distance but time .
The migrant families I study often reminisce about times when they lived together, recalling humorous incidents or past mishaps that lead to shared laughter. This isn’t just nostalgia: Separated relatives leverage these shared memories to imagine what it might be like to live together again.
For example, two sisters talk about how they once shared household tasks, using these memories to imagine a scenario in which the migrant sister could play with the young niece she’s never met.
The future is uncertain for migrant families. Many hope and plan to be together again, but restrictive immigration policies often prevent relatives from reuniting or even visiting each other .
For those locked down by coronavirus, the isolation should end in weeks or months. In the meantime, thoughtful communication can help sustain long-distance relationships.
And even after this crisis passes, I hope these lessons from migrant families will continue to enrich conversation and deepen social connections.
[ Insight, in your inbox each day. You can get it with The Conversation’s email newsletter .]
Lynnette Arnold , Assistant Professor of Anthropology, University of Massachusetts Amherst
This article is republished from The Conversation under a Creative Commons license. Read the original article .
Twitter and the way of the hashtag (by Jean Burgess for The Conversation)
Perhaps no single character has been as iconic a symbol of Twitter as the now-ubiquitous hashtag.
The syntax of the hashtag has a few simple rules: it consists of the hash symbol (#) immediately followed by a string of alphanumeric characters, with no spaces or punctuation.
It is used routinely in social media communication across a number of platforms including Tumblr, Instagram, and even Facebook, but its most important point of emergence and polarisation has been in Twitter.
The hashtag remains most comfortable in Twitter, and it was Twitter that turned it into a highly significant, multi-functional feature. The hashtag has made its way off the internet, appearing regularly on television, in advertising, on products and on protest signs around the world.
From its beginnings as a geeky tool designed to help individual users deal with an increasingly fragmented information stream, Twitter made the hashtag a new and powerful part of the world’s cultural, social and political vocabulary.
The @ feature helped people organise into pairs and create conversational streams. The hashtag, which organises tweets into topics, publics , and communities, goes to the heart of a crucial question: how is the internet organised and for whom?
Read more:
Anger is all the rage on Twitter when it’s cold outside (and on Mondays)
Adding value
Although its use on Twitter was new, the # has a prehistory both as a punctuation mark and as part of internet communication. Imported from elsewhere, as was the @, the hashtag brought some of its prior conventional understandings with it.
Known as the “octothorpe” by typography experts, in early computer-mediated communication the hash or pound symbol was used to mark channels and roles in systems like Internet Relay Chat (real-time, online text messaging used as early as 1988). It therefore worked to both categorise topics and group users.
The # also became closely tied to crowd-sourced content tagging systems. On the music-streaming site Last.fm , users could tag artists and songs. The site used these tags as information to “learn” about music, fuelling recommendations and radio streams, and laying the groundwork for Spotify and other apps’ current recommendation algorithms.
User-contributed tags were important on the Flickr photosharing website, where they helped direct people to images and to one another — a practice that was carried over to Instagram . Crucially, users could add as many tags to their Flickr photographs as they liked, creating a system that was less a taxonomy (an expertly ordered system based on exclusive, hierarchical categories) and more a “folksonomy” (a crowd-sourced one, based on inclusive tags and aggregation).
Folksonomical ordering, in the mid-2000s, was widely imagined as a more efficient, organic way of ordering content than categories or directories, and it was this model that underpinned the popular social bookmarking service del.icio.us .
The Flickr folksonomy of user-contributed tags was paradigmatic of the Web 2.0 ideology — marked by a shift from the web 1.0 idea that web development was about serving content to audiences to one where the goal was building architectures for participation of users (sometimes distinguished from passive website “visitors”) and the expectation that the user community’s activities would add further value.
Reddit’s systems for upvoting user-curated content, subreddits and modern Twitter’s aggregated trending topics are contemporary versions of this early tag-based co-curation model.
Read more:
Don’t just blame YouTube’s algorithms for ‘radicalisation’. Humans also play a part
A #solution to a problem
As far as we know, the hashtag’s use in Twitter was first proposed in mid-2007 by Chris Messina in a series of blog posts.
In Messina’s view, the hashtag was a solution to a need. At this time, it was still possible to see a public feed of every single tweet from a public account. Topical conversations among people who did not follow one another were incoherent at best.
The users advocating for the hashtag were technically proficient (many of them also developers) with an active online presence, who positioned themselves as participants in a community of lead users.
While some users were experimenting with hashtags, Messina’s vision for them didn’t catch on widely until a particularly acute and sufficiently significant event — the San Diego brushfires in 2007.
With this event, Messina achieved wider take-up of the hashtag as a tool for coordinating crisis communication by actively lobbying other lead users and media organisations.
https://twitter.com/nateritter/status/356487862
Journalist Nick Bilton relates an encounter between Twitter founders Biz Stone and Ev Williams and Messina, at the Twitter offices, as follows:
‘I really think you should do something with hashtags on Twitter,’ Chris told them. ‘Hashtags are for nerds,’ Biz replied. Ev added that they were ‘too harsh and no one is ever going to understand them.’
Culture clash
Twitter had begun wrestling with the problem (which still haunts it) of conflict between the cultures of expert users that made the platform work for them and the new users they alienated but whom the company badly needed to sustain its growth. The hashtag provoked contestation between Twitter’s different cultures as it was taken up both for the serious uses – such as disaster and professional discussion Messina had envisioned – and to create sociable rituals and play.
From the beginning, there was debate around the right way to use hashtags.
As Messina’s historical documentation and that of others show, there were several competing models of how and why to coordinate Twitter activity as the flow of tweets started to grow beyond an easily manageable size.
Perhaps the # was a tag, designed to help organise collections of tweets on shared topics? Or was it a way to form channels, or groups of users interested in those topics?
Underlying these different models of what the hashtag could become were different models of Twitter: as an information network, a social networking site or online community, or a platform for discussion and the emergence of publics (organised communities).
Such ideas were still new and hotly contested at the time. Though the informational seems to have won out over the conversational model of Twitter, the hashtag remains, and is used for an astonishing array of social, cultural, and political purposes — some of them vitally useful, not all of them serious, and some of them downright toxic.
The website Hashtags.org was launched in December 2007, and provided a real-time tracking and indexing of hashtags before Twitter implemented search. Participants at an event, for instance, could visit the website to see other tweets from the same event.
The hashtags in the earliest archived version of the Hashtags.org homepage, from April 2008, include a number of academic and tech conferences (#EconSM, #netc08, #interact2008) and sporting and entertainment events (#idol, #yankees, #REDSOX), and tweet categories (#haiku). Hashtags were used for coordinating discussion topics and finding like-minded users (#seriousgames, #punknews, #college, #PHX), brands and products (#gmail, #firefox), and even people (such as Wired journalist #ChrisAnderson).
Back then, the most tweeted hashtags were represented as amassing tweets numbering in the tens or at most hundreds, a reminder of the modest scale of Twitter at the time. Uses of hashtags, such as for humour, activism or second-screen television viewing, had yet to emerge.
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More than chatter
Ever since those early debates about whether Twitter needed “channels” (of topics) or “groups” (of users), hashtags have continued to play both structural and semantic roles: that is, they coordinate both communities and topics, helping users find each other and encounter a range of contributions to the discussion of issues and events.
The hashtag has fostered the rise of Twitter as a platform for news, information and professional promotion, yet the forces that allowed hashtags to become influential are deeply rooted in its conversational and sociable uses.
https://twitter.com/mcmsharksxx/status/1275793571107549187
The capacity of the hashtag to help people navigate real-time events such as disasters, protests and conferences, and to expand and solidify social connections and community, proved particularly ideal for social movements and activism.
Such uses have in many ways come to define both the hashtag and, increasingly, Twitter itself. Perhaps the most notable confluence of hashtags and bodies-in-the-street activism has come from #Blacklivesmatter. As US academics Deen Freelon, Charlton D. McIlwain, and Meredith D. Clark document :
The Twitter hashtag was created in July 2013 by activists Alicia Garza, Patrisse Cullors, and Opal Tometi in the wake of George Zimmerman’s acquittal for second-degree murder of unarmed Black teenager Trayvon Martin.
For more than a year, #Blacklivesmatter was only a hashtag, and not a very popular one: it was used in only 48 public tweets in June 2014 and in 398 tweets in July 2014. But by August 2014 that number had skyrocketed to 52,288, partly due to the slogan’s frequent use in the context of the Ferguson protests. Some time later, Garza, Cullors, Tometi, and others debuted Black Lives Matter as a chapter-based activist organization.
It’s easy to dismiss hashtag activism as a form of slacktivism rather than real political engagement. But the rise of #Blacklivesmatter and its ties to street protests and unjust policing serves as an important reminder of the embodiment and liveness of many events that might look merely like “data” or chatter when viewed as hashtags.
https://twitter.com/JoshuaPHilll/status/1277789201183002627
This is an edited extract from Twitter: A Biography by Jean Burgess and Nancy K. Baym, published by NYU Press.
Nancy K. Baym is Senior Principal Researcher, Microsoft Research and Research Affiliate in Comparative Media Studies/Writing, Massachusetts Institute of Technology, Cambridge Massachusetts. Jean Burgess , Professor and Director, Digital Media Research Centre, Queensland University of Technology
This article is republished from The Conversation under a Creative Commons license. Read the original article .
Media Attributions
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You might think intuitively that the recoil velocity should be exactly one fourth of a meter per second, and you'd be right except that the wagon has some mass as well. Our present approach, however, only requires that we give a way to test for equality of masses. To predict the recoil velocity from scratch, we'd need to use conservation of momentum, which is discussed in a later chapter.
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The argument only fails for objects light enough to be affected appreciably by air friction: a bunch of feathers falls differently if you wad them up because the pattern of air flow is altered by putting them together.
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V.B. Braginskii and V.I. Panov, Soviet Physics JETP 34, 463 (1972).
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The principle of Galilean relativity is extended on page 190.


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Acheulean tool industry
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the production of more complex tools, including hand axes, by
H. erectus
from 1.6 million to 200,000 years ago.
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archaic
Homo
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the period of time that precedes the emergence of the earliest early modern humans (
Homo sapiens
) around 300,000 years ago.
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biface tools
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a type of tool characteristic of the Acheulean tool industry, with both sides worked.
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Broca’s area
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a region in the frontal lobe of the brain (which includes two Brodmann areas) first found in
H. habilis
and connected with the production of speech.
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canine fossa
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a facial depression above the canine tooth found in modern humans.
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coevolution
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an interaction between different species that influences each species’ evolution; the simplest case of this is predator-prey relationships.
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cranial capacity
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the volume of the interior of the cranium or skull, providing an approximate size of the brain.
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encephalization
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increased brain size over time.
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encephalization quotient
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a measurement defined as the ratio between brain and body size.
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endocranial cast
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an impression taken from the inside of the cranium (braincase), frequently used by paleoanthropologists to determine the shape and approximate size of the brain in hominids and other primates.
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evolutionary mismatch
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a hypothesis that disease and nutritional deficiencies result when people’s bodies are unable to adapt to an environment that they have not spent most of their evolutionary history in.
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genome
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the complete set of genes or genetic material present in a cell or organism.
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ghost population
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proposed group for which no fossil evidence has yet been found.
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hafting
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the process of attaching stone points to a handle, which increases a tool’s effectiveness for hunting.
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handedness
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the use of a dominant hand, suggests lateralization of the brain and cognitive development.
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intentional burials
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evidence of placing the dead in a specific manner, suggesting ritualistic practice.
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island dwarfism
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mainland small animal species that colonize islands might evolve larger bodies if the island does not contain key predators. On the other hand, larger species may become smaller due to more limited resources in an island environment.
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lithic reduction
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the process of fashioning stones or rocks into tools or weapons by removing some parts.
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melanin
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substance that determines the color of skin pigmentation and protects people from ultraviolet radiation. Skin will have higher levels of melanin the closer to the equator one lives.
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Mitochondrial Eve
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genes traced through mitochondrial DNA that represent the female genetic originator of all humans who lived 200,000 years ago in Africa.
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Mousterian tool industry
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a complex stone tool technology largely associated with the Neanderthal.
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mtDNA
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the DNA located in the mitochondria that can be passed down unchanged from female genetic contributor to child.
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mtMRCA
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“mitochondrial most recent common ancestor,” or Mitochondrial Eve, representing the common ancestor of
H. sapiens
around 200,000 years ago.
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occipital bun
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an anatomical feature seen in the Neanderthal skull that appears in the rear of the skull.
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Oldowan tool Industry
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the oldest and most primitive tool industry; production and use are largely in association with
H. habilis.
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Out of Africa theory
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theory that proposes that
Homo sapiens
developed first in Africa and then spread around the world between 100,000 and 200,000 years ago.
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postcranial features
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skeletal material found in the body that is not related to the skull (cranial bones).
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sexual dimorphism
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differences in physical characteristics other than reproductive organs that appear between males and females of the same species.
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thermoregulation
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an adaptation that allows the body to control and regulate body temperature.


<IP_ADDRESS>: Categorizing Sources
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Once you have your research question, you’ll need information sources to answer it and meet the other information needs of your research project.
This section about categorizing sources will increase your sophistication about them and save you time in the long run because you’ll understand the “big picture”. That big picture will be useful as you plan your own sources for a specific research project, which we’ll help you with in the next section Sources and Information Needs.
You’ll usually have a lot of sources available to meet the information needs of your projects. In today’s complex information landscape, just about anything that contains information can be considered a potential source.
Here are a few examples:
With so many sources available, the question usually is not whether sources exist for your project but which ones will best meet your information needs.
Being able to categorize a source helps you understand the kind of information it contains, which is a big clue to (1) whether might meet one or more of your information needs and (2) where to look for it and similar sources.
A source can be categorized by:
As you may already be able to tell, sources can be in more than one category at the same time because the categories are not mutually exclusive.


43.7C: Contraception and Birth Control
- Describe the various methods of birth control and their associated failure rates
Contraception and Birth Control
Contraception, also known as birth control, is methods or devices used to prevent pregnancy. Strictly speaking, contraception aims to prevent the sperm and egg from joining, while birth control can refer to methods used to prevent a fertilized egg from developing into a fetus. Both terms are, however, frequently used interchangeably.
There are many methods of birth control, including barriers to sperm, hormones that prevent ovulation, sterilization procedures, and “natural” methods. Each method has an associated “failure rate”, which is the number of pregnancies resulting from the method’s use over a twelve-month period.
Barrier methods include those that prevent the sperm from reaching the egg. These include condoms, diaphragms, cervical caps, sponges, and spermicides; they have a failure rate of 15-24%. Barrier methods such as condoms, cervical caps, and diaphragms serve to block sperm from entering the uterus, thereby preventing fertilization. Chemicals such as spermicides, which are designed to kill sperm, are often used in conjunction; sponges, for example, are saturated with spermicides and are placed in the vagina at the cervical opening. Combinations of spermicidal chemicals and barrier methods achieve lower failure rates than do the methods when used separately.
Hormonal methods use synthetic progesterone (sometimes in combination with estrogen) to inhibit the hypothalamus from releasing FSH or LH, preventing an egg from being available for fertilization. The method of administering the hormone affects failure rate, although, in general, hormonal methods have a failure rate of 8%. The most reliable method, with a failure rate of less than 1 percent, is the implantation of the hormone under the skin. The same rate can be achieved through the sterilization procedures of vasectomy in the man or of tubal ligation in the woman, or by using an intrauterine device (IUD). IUDs are inserted into the uterus where they establish an inflammatory condition that prevents fertilized eggs from implanting into the uterine wall.
Sterilization is a one-time, permanent, surgical procedure. In a vasectomy, the vasa deferentia of a male are severed and then tied/sealed in a manner that prevents sperm from entering into the seminal stream (ejaculate). Tubal ligation or tubectomy is a surgical procedure for sterilization in which a woman’s fallopian tubes are clamped and blocked, or severed and sealed; either method prevents eggs from reaching the uterus for fertilization. Both of these procedures have a less than 1% failure rate.
 Nearly 25% of the couples using natural family planning or withdrawal can expect a failure of the method. Natural family planning is based on the monitoring of the menstrual cycle and having intercourse only during times when the egg is not available. A woman’s body temperature may rise a degree Celsius at ovulation and the cervical mucus may increase in volume, becoming more pliable. These changes give a general indication of when intercourse is more or less likely to result in fertilization.  This is, therefore, a risky method. The high failure rate is due to the possible presence of sperm in the bulbourethral gland’s secretion, which may enter the vagina prior to removing the penis.
Termination of an existing pregnancy can be spontaneous or voluntary. Spontaneous termination in the first trimester is referred to as spontaneous abortion, while in the second and third trimesters is referred to as a miscarriage. Spontaneous abortion usually occurs very early in the pregnancy (typically within the first few weeks). Pregnancy loss occurs when the embryo/fetus cannot develop properly so the gestation is naturally terminated. Voluntary termination of a pregnancy is called an elective abortion. Laws regulating elective abortion vary between states; the laws tend to view fetal viability as the criteria for allowing or preventing the procedure.
Key Points
- Barrier methods include those that prevent the sperm from reaching the egg ( condoms, diaphragms, cervical caps, sponges, and spermicides ); these methods have a failure rate of 15-24%.
- Hormonal methods include those that prevent a woman from ovulating (oral birth control pills, hormone injections, and hormone implants); these have a failure rate of 8%.
- Sterilization, such as a tubal ligation in a woman or a vasectomy in a man, is an extremely effective (although permanent) method of contraception, with a failure rate of 1%.

Key Terms
- condom : a flexible sleeve made of latex or other impermeable material such as sheepskin, worn over an erect penis during intercourse as a contraceptive or as a way to prevent the spread of STDs
- intrauterine device : a contraceptive device consisting of a spiral or similar shape of plastic or metal inserted through the vagina into the uterus in order to prevent the implantation of a fertilized egg
- vasectomy : the surgical removal of all or part of the vas deferens, usually as a means of male sterilization
- contraception : the use of a device or procedure to prevent conception as a result of sexual activity
- spermicide : a substance used for killing sperm


1.3: What is Biological Anthropology?
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Anthropology is the scientific study of humankind. The main purpose of anthropology is for us to understand human biological and cultural diversity, and the origins of humans. So, the humans that anthropologists study can be modern or ancient, and can span across the globe. Anthropologists are unique in that they study the entirety of human existence over time and space.
Figure \(\PageIndex{1}\) - Pride parade, Seattle, WA, June 26, 2011
Basic tenets of anthropology:
- Holism : Holism means that a part of something can only truly be understood if examined within relation to the whole of it. For anthropologists, this means that they try to understand humankind through the interrelationships of all aspects of human existence -- for example, human biology has to be examined within the context of human cultures and vice versa. In addition, all of this must be examined within the context of the environment and historical processes. In an effort to be holistic, anthropology is often an interdisciplinary field that crosses over into other fields such as history, geology, and ecology.
- Relativism : Relativism means that judgments, truths, or moral values have no absolutes, and can only be understood relative to the situation or individuals involved. For anthropologists, this means that they accept that all cultures are of equal value and must be studied from a neutral point of view. A good anthropologist must disregard their own beliefs, morals, and judgments when examining another culture. They must, instead, examine each culture within the context of its own beliefs.
- Universalism : Universalism means that whatever the theoretical principle is, it's equally applicable to all. For anthropologists, universalism means that we believe all humans are equal -- in intelligence, complexity, etc.This is in contrast to ethnocentrism , which is the belief that some peoples are more important or culturally/biologically better than other peoples.
- Culture : All humans have culture. Culture is the set of learned behaviors and knowledge that belong to a certain set of people. This is different from genetically hardwired behaviors (such as reflexes) in that they aren't biologically inherited. The most important thing to remember is that culture is learned.
Subfields in Anthropology
There are four subfields in anthropology: cultural anthropology, biological (or physical) anthropology, archaeology, and linguistic anthropology.
- Sociocultural Anthropology
Sociocultural anthropologists examine social patterns and practices across cultures, with a special interest in how people live in particular places and how they organize, govern, and create meaning. A hallmark of sociocultural anthropology is its concern with similarities and differences, both within and among societies, and its attention to race, sexuality, class, gender, and nationality. Research in sociocultural anthropology is distinguished by its emphasis on participant observation, which involves placing oneself in the research context for extended periods of time to gain a first-hand sense of how local knowledge is put to work in grappling with practical problems of everyday life and with basic philosophical problems of knowledge, truth, power, and justice. Topics of concern to sociocultural anthropologists include such areas as health, work, ecology and environment, education, agriculture and development, and social change.
- Biological (or Physical) Anthropology
Biological anthropologists seek to understand how humans adapt to diverse environments, how biological and cultural processes work together to shape growth, development and behavior, and what causes disease and early death. In addition, they are interested in human biological origins, evolution and variation. They give primary attention to investigating questions having to do with evolutionary theory, our place in nature, adaptation and human biological variation. To understand these processes, biological anthropologists study other primates (primatology), the fossil record (paleoanthropology), prehistoric people (bioarchaeology), and the biology (e.g., health, cognition, hormones, growth and development) and genetics of living populations.
- Archaeology
Archaeologists study past peoples and cultures, from the deepest prehistory to the recent past, through the analysis of material remains, ranging from artifacts and evidence of past environments to architecture and landscapes. Material evidence, such as pottery, stone tools, animal bone, and remains of structures, is examined within the context of theoretical paradigms, to address such topics as the formation of social groupings, ideologies, subsistence patterns, and interaction with the environment. Like other areas of anthropology, archaeology is a comparative discipline; it assumes basic human continuities over time and place, but also recognizes that every society is the product of its own particular history and that within every society there are commonalities as well as variation.
- Linguistic Anthropology
Linguistic anthropology is the comparative study of ways in which language reflects and influences social life. It explores the many ways in which language practices define patterns of communication, formulate categories of social identity and group membership, organize large-scale cultural beliefs and ideologies, and, in conjunction with other forms of meaning-making, equip people with common cultural representations of their natural and social worlds. Linguistic anthropology shares with anthropology in general a concern to understand power, inequality, and social change, particularly as these are constructed and represented through language and discourse.
(From: American Anthropological Association. 2015. What is Anthropology? < http://www.aaanet.org/about/WhatisAnthropology.cfm >. Accessed 2015 May 7.)
While the sheer scope of anthropological study requires people to choose a subfield and specialization within that subfield many anthropologists integrate multiple subfields in their work. It is common to find biological anthropologists working with cultural anthropologists to examine things such as disease patterns and archaeologists to look at migration patterns in the past.
What is Biological Anthropology?
Figure \(\PageIndex{2}\): Heather Hecht Edgar (University of New Mexico) conducting research at the Czech National Museum, 2005
Biological anthropology is the study of the biology of humans and their nearest biological relatives. "Biological relatives" include both humankind's ancestors and our nearest living relatives, the non-human primates. Biological anthropologists fall under 3 major categories:
- Human biology (human biological diversity, genetics, adaptations to environmental stressors, etc.)
- Primatology (non-human primate biology, evolution, behavior, ecology, etc.)
- Paleoanthropology (human origins and human evolution)
These are the topics we'll be studying in this course. Each unit follows each of the 3 major categories found in biological anthropology.
- Unit 1: In this unit, you'll learn the basic toolkit you'll need (evolutionary theory) in order to understand the rest of the course. This unit also covers human biology.
- Unit 2: This unit covers primatology.
- Unit 3: This unit covers paleoanthropology.


3.3: The Newborn
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Assessing the Newborn
The Apgar assessment is conducted one minute and five minutes after birth. This is a very quick way to assess the newborn's overall condition. Five measures are assessed: Heart rate, respiration, muscle tone (assessed by touching the baby's palm), reflex response (the Babinski reflex is tested), and color. A score of 0 to 2 is given on each feature examined. An Apgar of 5 or less is cause for concern. The second Apgar should indicate improvement with a higher score. 22
Another way to assess the condition of the newborn is the Neonatal Behavioral Assessment Scale (NBAS). The baby's motor development, muscle tone, and stress response are assessed. This tool has been used around the world to further assess the newborn, especially those with low Apgar scores, and to make comparisons of infants in different cultures (Brazelton & Nugent, 1995).
Newborns are also routinely screened for different conditions. Within the first 24 to 48 hours after birth, babies born in hospitals undergo a simple heel stick and a few drops of blood are collected on a special paper card. Providers test those dried blood spots for a variety of different congenital disorders, or conditions that are present when the baby is born. In California, newborns are now screened for 80 different genetic and congenital disorders.
Newborns are also screened for hearing disorders and certain serious heart problems using methods other than dried blood spots. 25
Problems of the Newborn
Anoxia
Anoxia is a temporary lack of oxygen to the brain. Difficulty during delivery may lead to anoxia which can result in brain damage or in severe cases, death. Babies who suffer both low birth weight and anoxia are more likely to suffer learning disabilities later in life as well.
Low Birth Weight
A child is considered low birth weight if he or she weighs less than 5 pounds 8 ounces (2500 grams). About 8.2 percent of babies born in the United States are of low birth weight (Center for Disease Control, 2015a). Sixty-seven percent of these babies are also preterm.
A low birth weight baby has difficulty maintaining adequate body temperature because it lacks the fat that would otherwise provide insulation. Such a baby is also at more risk for infection. Very low birth weight babies (2 pounds or less) have an increased risk of developing cerebral palsy. Many causes of low birth weight are preventable with proper prenatal care.
Preterm
A newborn might also have a low birth weight if it is born at less than 37 weeks gestation, which qualifies it as a preterm baby (CDC, 2015c). Early birth can be triggered by anything that disrupts the mother's system. For instance, vaginal infections can lead to premature birth because such infection causes the mother to release anti-inflammatory chemicals which, in turn, can trigger contractions. Smoking and the use of other teratogens can lead to preterm birth. A significant consequence of preterm birth includes respiratory distress syndrome, which is characterized by weak and irregular breathing (see the image below). Premature babies often cannot yet regulate their own temperature or feed by nursing or bottle. They may struggle to regulate their heart rate effectively and may experience jaundice. They often require care in the Neonatal Intensive Care Unit (NICU) until they are as healthy as a full-term baby.
Small-for-Date Infants
Infants that have birth weights that are below expectation based on their gestational age are referred to as small-for-date . These infants may be full term or preterm (see image below), but still weigh less than 90% of all babies of the same gestational age. This is a very serious situation for newborns as their growth was adversely affected. Regev et al. (2003) found that small-for-date infants died at rates more than four times higher than other infants.
Postmature
When babies are not born by 42 weeks gestation, or two weeks after their due date, they are considered overdue or postmature . There are some concerns about how long the placenta can function and most doctors will consider induction for overdue babies.
Stillborn
When a fetus (unborn baby) dies while still inside the mother (after 20-24 weeks gestation) or dies during delivery (childbirth). It is said that the delivered baby is stillborn. The causes of many stillbirths are unknown, even when special tests are done to learn the cause. Possible causes include: nicotine, alcohol, or drugs taken by the mother during pregnancy, physical trauma, radiation poisoning, Rh disease, and umbilical cord problems. The number of stillbirths in the United States is about 1 in 115 births, which is about 26,000 a year, or one every 20 minutes. 28
Characteristics of Newborns
Size
The average newborn in the United States weighs about 7.5 pounds and is about 20 inches in length. For the first few days of life, infants typically lose about 5 percent of their body weight as they eliminate waste and get used to feeding. This often goes unnoticed by most parents, but can be cause for concern for those who have a smaller infant. This weight loss is temporary, however, and is followed by a rapid period of growth.
Body Proportions
The head initially makes up about 50 percent of our entire length when we are developing in the womb. At birth, the head makes up about 25 percent of our length (think about how much of your length would be head if the proportions were still the same!).
Brain Development
Some of the most dramatic physical change that occurs during this period is in the brain. At birth, the brain is about 25 percent its adult weight and this is not true for any other part of the body. While most of the brain’s 100 to 200 billion neurons are present at birth, they are not fully mature. During the next several years dendrites or connections between neurons will undergo a period of transient exuberance or temporary dramatic growth. 31
Appearance at Birth
During labor and birth, the infant's skull changes shape to fit through the birth canal, sometimes causing the child to be born with a misshapen or elongated head. It will usually return to normal on its own within a few days or weeks.
Some newborns have a fine, downy body hair called lanugo. It may be particularly noticeable on the back, shoulders, forehead, ears and face of premature infants. Lanugo disappears within a few weeks. Likewise, not all infants are born with lush heads of hair. Some may be nearly bald while others may have very fine, almost invisible hair. Some babies are even born with a full head of hair. Amongst fair-skinned parents, this fine hair may be blond, even if the parents are not. The picture on the left shows lanugo on the shoulders of newborn twins.
Immediately after birth, a newborn's skin is often grayish to dusky blue in color. As soon as the newborn begins to breathe, usually within a minute or two, the skin's color returns to its normal tone. Newborns are wet, covered in streaks of blood, and coated with a white substance known as vernix , which is thought to act as an antibacterial barrier, seen in the picture on the right.
The scalp may also be temporarily bruised or swollen, especially in hairless newborns, and the area around the eyes may be puffy.
The newborn may also have Mongolian spots (blue or blue black birthmark on the lower back), various other birthmarks, or peeling skin, particularly on the wrists, hands, ankles, and feet. 34
A newborn's genitals are enlarged and reddened, with male infants having an unusually large scrotum. The breasts may also be enlarged, even in male infants. This is caused by naturally-occurring maternal hormones and is a temporary condition.
The umbilical cord of a newborn is bluish-white in color. After birth, the umbilical cord is normally cut, leaving a 1–2 inch stub. The umbilical stub will dry out, shrivel, darken, and spontaneously fall off within about 3 weeks. Occasionally, hospitals may apply triple dye to the umbilical stub to prevent infection, which may temporarily color the stub and surrounding skin purple.
Newborns lose many of the above physical characteristics quickly. Thus older babies look very different. While older babies are considered "cute," newborns can be "unattractive" by the same criteria and first time parents may need to be educated in this regard. 36
Sleep
A newborn typically sleeps approximately 16.5 hours per 24-hour period. The infant sleeps in several periods throughout the day and night, which means they wake often throughout the day and night. (Salkind, 2005). 37
Reflexes
Newborns are equipped with a number of reflexes , which are involuntary movements in response to stimulation. Some of the more common reflexes, such as the sucking reflex and rooting reflex, are important to feeding. The grasping and stepping reflexes are eventually replaced by more voluntary behaviors. Within the first few months of life these reflexes disappear, while other reflexes, such as the eye-blink, swallowing, sneezing, gagging, and withdrawal reflex stay with us as they continue to serve important functions. 39
Sensory Capacities
Throughout much of history, the newborn was considered a passive, disorganized being who possessed minimal abilities. However, current research techniques have demonstrated just how developed the newborn is with especially organized sensory and perceptual abilities.
Vision
The womb is a dark environment void of visual stimulation. Consequently, vision is the most poorly developed sense at birth and time is needed to build those neural pathways between the eye and the brain. Newborns typically cannot see further than 8 to 16 inches away from their faces, and their visual acuity is about 20/400, which means that an infant can see something at 20 feet that an adult with normal vision could see at 400 feet. Thus, the world probably looks blurry to young infants.
Hearing
The infant’s sense of hearing is very keen at birth, and the ability to hear is evidenced as soon as the 7th month of prenatal development. In fact, an infant can distinguish between very similar sounds as early as one month after birth and can distinguish between a familiar and unfamiliar voice even earlier. Infants are especially sensitive to the frequencies of sounds in human speech and prefer the exaggeration of infant-directed speech, which will be discussed later. Newborns also prefer their mother’s voices over another female when speaking the same material (DeCasper & Fifer, 1980). Additionally, they will register in utero specific information heard from their mother’s voice. 41
Early Hearing
DeCasper and Spence (1986) tested 16 infants whose mothers had previously read to them prenatally. The mothers read several passages to their fetuses, including the first 28 paragraphs of The Cat in the Hat, beginning when they were 7 months pregnant. The fetuses had been exposed to the stories on average of 67 times or 3.5 hours.
During the testing, the infants were able to choose between recordings of two stories, one of which was a story their mothers read to them while in the womb, based on how fast they sucked on their pacifiers. They showed a preference for the stories that their mothers read to them while in the womb. 42
Touch and Pain
Immediately after birth, a newborn is sensitive to touch and temperature, and is also highly sensitive to pain, responding with crying and cardiovascular responses (Balaban & Reisenauer, 2013). Newborns who are circumcised, which is the surgical removal of the foreskin of the penis, without anesthesia experience pain as demonstrated by increased blood pressure, increased heart rate, decreased oxygen in the blood, and a surge of stress hormones (United States National Library of Medicine, 2016). Research has demonstrated that infants who were circumcised without anesthesia experienced more pain and fear during routine childhood vaccines. Fortunately, many circumcisions are now done with the use of local anesthetics.
Taste and Smell
Studies of taste and smell demonstrate that babies respond with different facial expressions, suggesting that certain preferences are innate. Newborns can distinguish between sour, bitter, sweet, and salty flavors and show a preference for sweet flavors. Newborns also prefer the smell of their mothers. An infant only 6 days old is significantly more likely to turn toward its own mother’s breast pad than to the breast pad of another baby’s mother (Porter, Makin, Davis, & Christensen, 1992), and within hours of birth an infant also shows a preference for the face of its own mother (Bushnell, 2001; Bushnell, Sai, & Mullin, 1989).
Infants seem to be born with the ability to perceive the world in an intermodal way; that is, through stimulation from more than one sensory modality. For example, infants who sucked on a pacifier with a smooth surface preferred looking at visual models of a pacifier with a smooth surface. But those that were given a pacifier with a textured surface preferred to look at a visual model of a pacifier with a textured surface. 44
Contributors and Attributions
22. Lifespan Development: A Psychological Perspective (page 63) by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0
25. Newborn Screening is in the public domain ; Newborn Screening Program (NBS) by the California Department of Public Health is in the public domain
28. Lifespan Development: A Psychological Perspective (pages 62-63) by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0 ; Stillbirth by Wikipedia by Wikipedia is licensed under CC BY-SA 3.0
31. Lifespan Development - Chapter 4: Infancy - Physical Development by Lumen Learning references Psyc 200 Lifespan Psychology by Laura Overstreet, licensed under CC BY 4.0
34. Mongolian Spot by Wiktionary is licensed under CC BY-SA 3.0
36. Infant by WikiDoc is licensed under CC BY-SA 3.0
37. Lifespan Development: A Psychological Perspective (page 71) by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0
39. Lifespan Development: A Psychological Perspective (page 73) by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0
41. Lifespan Development: A Psychological Perspective (pages 76-77) by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0
42. Lifespan Development: A Psychological Perspective (page 76) by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0
44. Lifespan Development: A Psychological Perspective (pages 76-77) by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0


2.2: Assignment- Exploring Earth’s layers and seismic-wave travel times
Module 2 Assignment
Exploring Earth’s layers and seismic-wave travel times
Overview
Earth’s interior layers can be distinguished in two ways: by their chemical composition or by their physical characteristics and the way they “move” (in a rigid or a “plastic” manner, for instance). Sometimes the different terms we use to describe the interior are confused with one another, or they are treated as interchangeable terms, when really they are quite different. For example, the terms crust, mantle, and core are differentiated by their chemical composition, while the terms lithosphere and asthenosphere are differentiated by their physical (mechanical) properties. However, the terms crust and lithosphere are often (and incorrectly) used interchangeably.
Instructions
Part 1
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Consider the following terms that are used to describe various layers of Earth’s interior:
- Crust
- Continental crust
- Oceanic crust
- Lithosphere
- Asthenosphere
- Mantle
- Mesosphere
- Core
- Outer core (liquid)
- Inner core (solid)
How might you hand-draw a figure for a textbook that includes all of these components of Earth’s interior? In other words, how might you reconcile these components of Earth’s interior and the two different ways they are characterized (chemical vs. physical/mechanical properties) in a single illustration?
- Write a paragraph (at least 5 – 6 sentences) that summarizes what tools scientists use to determine the depths at which these layers are situated in Earth’s interior.
Part 2
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The March 11th, 2011 earthquake in Japan was a magnitude 9.1— one of the most powerful earthquakes to ever be recorded on Earth. Let’s assume the P-waves traveled outward from the focus of the earthquake at a rate of 5 km/s, and the S-waves traveled outward at a rate of 4 km/s.
- Which waves, P or S, would arrive at seismograph stations first?
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S-waves are more damaging than P-waves, and surface waves are the most damaging seismic waves of all.How much warning did residents of the following cities have before the more serious shaking began, starting with arrival of the S-waves?
(Hint: Calculate the difference in arrival time between the P- and S-waves in each city.)
Show all your work.
- Sendai (130 km from epicenter)
- Tokyo (373 km from epicenter)
- Osaka (770 km from epicenter)
Grading
10 points : Accurately responded to each question, including a clearly drawn figure of Earth’s interior, a well-written brief summary, and showing the math for each of the 3 calculations. Any sources used were cited.
8 points : Responded to each question, but may have addressed one or two incorrectly.
5 points : More than two questions missed or answered incorrectly; sources were not cited.
2 points : Only very partial information provided.
0 points : Did not complete the assignment.


11: Sea Level
In the simplest sense, sea level is the elevation of the surface of the ocean. But when we think about it in the geologic sense, things quickly become more complicated as we must consider variations in space, changes in time, and the complex interactions between the surface of the ocean, the Earth's crust, and the rate at which sediment is supplied to an area. In this chapter we will briefly explore all of these topics and think about how they are preserved in the geologic record.
- Explain what happens during transgressions and regressions.
- Define water depth, eustatic sea level, and relative sea level.
- Explain how tectonism and eustasy interact to cause increases or decreases in relative sea level.
Chapter thumbnail shows a relative sea level curve ( Page Quinton via Wikimedia Commons ; CC BY-SA 4.0 ).
-
- 11.2: Sea Level Terminology
- Clear terminology is an important precondition for a meaningful conversation about "sea level". Water depth is an instantaneous, location specific measurement of the distance between the water surface and the sediment surface. Eustatic sea level is the distance between the center of the Earth and the sea surface. Relative sea level is the distance between a local datum and the sea surface; it is a function of tectonism and eustasy and can be tracked through time.


14.12: Exercises
Write equations that show NH 3 as both a conjugate acid and a conjugate base.
Write equations that show acting both as an acid and as a base.
Show by suitable net ionic equations that each of the following species can act as a Brønsted-Lowry acid:
(a)
(b) HCl
(c) NH 3
(d) CH 3 CO 2 H
(e)
(f)
Show by suitable net ionic equations that each of the following species can act as a Brønsted-Lowry acid:
(a) HNO 3
(b)
(c) H 2 S
(d) CH 3 CH 2 COOH
(e)
(f) HS −
Show by suitable net ionic equations that each of the following species can act as a Brønsted-Lowry base:
(a) H 2 O
(b) OH −
(c) NH 3
(d) CN −
(e) S 2−
(f)
Show by suitable net ionic equations that each of the following species can act as a Brønsted-Lowry base:
(a) HS −
(b)
(c)
(d) C 2 H 5 OH
(e) O 2−
(f)
What is the conjugate acid of each of the following? What is the conjugate base of each?
(a) OH −
(b) H 2 O
(c)
(d) NH 3
(e)
(f) H 2 O 2
(g) HS −
(h)
What is the conjugate acid of each of the following? What is the conjugate base of each?
(a) H 2 S
(b)
(c) PH 3
(d) HS −
(e)
(f)
(g) H 4 N 2
(h) CH 3 OH
Identify and label the Brønsted-Lowry acid, its conjugate base, the Brønsted-Lowry base, and its conjugate acid in each of the following equations:
(a)
(b)
(c)
(d)
(e)
(f)
(g)
Identify and label the Brønsted-Lowry acid, its conjugate base, the Brønsted-Lowry base, and its conjugate acid in each of the following equations:
(a)
(b)
(c)
(d)
(e)
(f)
(g)
What are amphiprotic species? Illustrate with suitable equations.
State which of the following species are amphiprotic and write chemical equations illustrating the amphiprotic character of these species:
(a) H 2 O
(b)
(c) S 2−
(d)
(e)
State which of the following species are amphiprotic and write chemical equations illustrating the amphiprotic character of these species.
(a) NH 3
(b)
(c) Br −
(d)
(e)
Is the self-ionization of water endothermic or exothermic? The ionization constant for water ( K w ) is 2.9 10 −14 at 40 °C and 9.3 10 −14 at 60 °C.
Explain why a sample of pure water at 40 °C is neutral even though [H 3 O + ] = 1.7 10 −7 M . K w is 2.9 10 −14 at 40 °C.
The ionization constant for water ( K w ) is 2.9 10 −14 at 40 °C. Calculate [H 3 O + ], [OH − ], pH, and pOH for pure water at 40 °C.
The ionization constant for water ( K w ) is 9.311 10 −14 at 60 °C. Calculate [H 3 O + ], [OH − ], pH, and pOH for pure water at 60 °C.
Calculate the pH and the pOH of each of the following solutions at 25 °C for which the substances ionize completely:
(a) 0.200 M HCl
(b) 0.0143 M NaOH
(c) 3.0 M HNO 3
(d) 0.0031 M Ca(OH) 2
Calculate the pH and the pOH of each of the following solutions at 25 °C for which the substances ionize completely:
(a) 0.000259 M HClO 4
(b) 0.21 M NaOH
(c) 0.000071 M Ba(OH) 2
(d) 2.5 M KOH
What are the pH and pOH of a solution of 2.0 M HCl, which ionizes completely?
What are the hydronium and hydroxide ion concentrations in a solution whose pH is 6.52?
Calculate the hydrogen ion concentration and the hydroxide ion concentration in wine from its pH. See Figure 14.2 for useful information.
Calculate the hydronium ion concentration and the hydroxide ion concentration in lime juice from its pH. See Figure 14.2 for useful information.
The hydronium ion concentration in a sample of rainwater is found to be 1.7 10 −6 M at 25 °C. What is the concentration of hydroxide ions in the rainwater?
The hydroxide ion concentration in household ammonia is 3.2 10 −3 M at 25 °C. What is the concentration of hydronium ions in the solution?
Explain why the neutralization reaction of a strong acid and a weak base gives a weakly acidic solution.
Explain why the neutralization reaction of a weak acid and a strong base gives a weakly basic solution.
Use this list of important industrial compounds (and Figure 14.8) to answer the following questions regarding: Ca(OH) 2 , CH 3 CO 2 H, HCl, H 2 CO 3 , HF, HNO 2 , HNO 3 , H 3 PO 4 , H 2 SO 4 , NH 3 , NaOH, Na 2 CO 3 .
(a) Identify the strong Brønsted-Lowry acids and strong Brønsted-Lowry bases.
(b) Identify the compounds that can behave as Brønsted-Lowry acids with strengths lying between those of H 3 O + and H 2 O.
(c) Identify the compounds that can behave as Brønsted-Lowry bases with strengths lying between those of H 2 O and OH − .
The odor of vinegar is due to the presence of acetic acid, CH 3 CO 2 H, a weak acid. List, in order of descending concentration, all of the ionic and molecular species present in a 1- M aqueous solution of this acid.
Household ammonia is a solution of the weak base NH 3 in water. List, in order of descending concentration, all of the ionic and molecular species present in a 1- M aqueous solution of this base.
Explain why the ionization constant, K a , for H 2 SO 4 is larger than the ionization constant for H 2 SO 3 .
Explain why the ionization constant, K a , for HI is larger than the ionization constant for HF.
Gastric juice, the digestive fluid produced in the stomach, contains hydrochloric acid, HCl. Milk of Magnesia, a suspension of solid Mg(OH) 2 in an aqueous medium, is sometimes used to neutralize excess stomach acid. Write a complete balanced equation for the neutralization reaction, and identify the conjugate acid-base pairs.
Nitric acid reacts with insoluble copper(II) oxide to form soluble copper(II) nitrate, Cu(NO 3 ) 2 , a compound that has been used to prevent the growth of algae in swimming pools. Write the balanced chemical equation for the reaction of an aqueous solution of HNO 3 with CuO.
What is the ionization constant at 25 °C for the weak acid the conjugate acid of the weak base CH 3 NH 2 , K b = 4.4 10 −4 .
What is the ionization constant at 25 °C for the weak acid the conjugate acid of the weak base (CH 3 ) 2 NH, K b = 5.9 10 −4 ?
Which base, CH 3 NH 2 or (CH 3 ) 2 NH, is the stronger base? Which conjugate acid, or , is the stronger acid?
Which is the stronger acid, or HBrO?
Which is the stronger base, (CH 3 ) 3 N or
Predict which acid in each of the following pairs is the stronger and explain your reasoning for each.
(a) H 2 O or HF
(b) B(OH) 3 or Al(OH) 3
(c) or
(d) NH 3 or H 2 S
(e) H 2 O or H 2 Te
Predict which compound in each of the following pairs of compounds is more acidic and explain your reasoning for each.
(a) or
(b) NH 3 or H 2 O
(c) PH 3 or HI
(d) NH 3 or PH 3
(e) H 2 S or HBr
Rank the compounds in each of the following groups in order of increasing acidity or basicity, as indicated, and explain the order you assign.
(a) acidity: HCl, HBr, HI
(b) basicity: H 2 O, OH − , H − , Cl −
(c) basicity: Mg(OH) 2 , Si(OH) 4 , ClO 3 (OH) (Hint: Formula could also be written as HClO 4 .)
(d) acidity: HF, H 2 O, NH 3 , CH 4
Rank the compounds in each of the following groups in order of increasing acidity or basicity, as indicated, and explain the order you assign.
(a) acidity: NaHSO 3 , NaHSeO 3 , NaHSO 4
(b) basicity:
(c) acidity: HOCl, HOBr, HOI
(d) acidity: HOCl, HOClO, HOClO 2 , HOClO 3
(e) basicity: HS − , HTe − ,
(f) basicity: BrO − ,
Both HF and HCN ionize in water to a limited extent. Which of the conjugate bases, F − or CN − , is the stronger base?
The active ingredient formed by aspirin in the body is salicylic acid, C 6 H 4 OH(CO 2 H). The carboxyl group (−CO 2 H) acts as a weak acid. The phenol group (an OH group bonded to an aromatic ring) also acts as an acid but a much weaker acid. List, in order of descending concentration, all of the ionic and molecular species present in a 0.001- M aqueous solution of C 6 H 4 OH(CO 2 H).
Are the concentrations of hydronium ion and hydroxide ion in a solution of an acid or a base in water directly proportional or inversely proportional? Explain your answer.
What two common assumptions can simplify calculation of equilibrium concentrations in a solution of a weak acid or base?
Which of the following will increase the percent of NH 3 that is converted to the ammonium ion in water?
(a) addition of NaOH
(b) addition of HCl
(c) addition of NH 4 Cl
Which of the following will increase the percentage of HF that is converted to the fluoride ion in water?
(a) addition of NaOH
(b) addition of HCl
(c) addition of NaF
What is the effect on the concentrations of HNO 2 , and OH − when the following are added to a solution of KNO 2 in water:
(a) HCl
(b) HNO 2
(c) NaOH
(d) NaCl
(e) KNO
What is the effect on the concentration of hydrofluoric acid, hydronium ion, and fluoride ion when the following are added to separate solutions of hydrofluoric acid?
(a) HCl
(b) KF
(c) NaCl
(d) KOH
(e) HF
Why is the hydronium ion concentration in a solution that is 0.10 M in HCl and 0.10 M in HCOOH determined by the concentration of HCl?
From the equilibrium concentrations given, calculate K a for each of the weak acids and K b for each of the weak bases.
(a) CH
3
CO
2
H:
= 1.34
10
−3
M
;
= 1.34
10
−3
M
;
[CH 3 CO 2 H] = 9.866 10 −2 M ;
(b) ClO − : [OH − ] = 4.0 10 −4 M ;
[HClO] = 2.38 10 −4 M ;
[ClO − ] = 0.273 M ;
(c) HCO
2
H: [HCO
2
H] = 0.524
M
;
= 9.8
10
−3
M
;
= 9.8
10
−3
M
;
(d) = 0.233 M ;
[C
6
H
5
NH
2
] = 2.3
10
−3
M
;
= 2.3
10
−3
M
From the equilibrium concentrations given, calculate K a for each of the weak acids and K b for each of the weak bases.
(a) NH
3
: [OH
−
] = 3.1
10
−3
M
;
= 3.1
10
−3
M
;
[NH 3 ] = 0.533 M ;
(b) HNO
2
:
= 0.011
M
;
= 0.0438
M
;
[HNO 2 ] = 1.07 M ;
(c) (CH
3
)
3
N: [(CH
3
)
3
N] = 0.25
M
;
[(CH
3
)
3
NH
+
] = 4.3
10
−3
M
;
[OH − ] = 3.7 10 −3 M ;
(d) = 0.100 M ;
[NH
3
] = 7.5
10
−6
M
;
[H
3
O
+
] = 7.5
10
−6
M
Determine K b for the nitrite ion, In a 0.10- M solution this base is 0.0015% ionized.
Determine K a for hydrogen sulfate ion, In a 0.10- M solution the acid is 29% ionized.
Calculate the ionization constant for each of the following acids or bases from the ionization constant of its conjugate base or conjugate acid:
(a) F −
(b)
(c)
(d)
(e)
(f) (as a base)
Calculate the ionization constant for each of the following acids or bases from the ionization constant of its conjugate base or conjugate acid:
(a) HTe − (as a base)
(b)
(c) (as a base)
(d) (as a base)
(e)
(f) (as a base)
Using the K a value of 1.4
Calculate the concentration of all solute species in each of the following solutions of acids or bases. Assume that the ionization of water can be neglected, and show that the change in the initial concentrations can be neglected.
(a) 0.0092 M HClO, a weak acid
(b) 0.0784 M C 6 H 5 NH 2 , a weak base
(c) 0.0810 M HCN, a weak acid
(d) 0.11 M (CH 3 ) 3 N, a weak base
(e) 0.120 M a weak acid, K a = 1.6 10 −7
Propionic acid, C 2 H 5 CO 2 H ( K a = 1.34 10 −5 ), is used in the manufacture of calcium propionate, a food preservative. What is the pH of a 0.698- M solution of C 2 H 5 CO 2 H?
White vinegar is a 5.0% by mass solution of acetic acid in water. If the density of white vinegar is 1.007 g/cm 3 , what is the pH?
The ionization constant of lactic acid, CH 3 CH(OH)CO 2 H, an acid found in the blood after strenuous exercise, is 1.36 10 −4 . If 20.0 g of lactic acid is used to make a solution with a volume of 1.00 L, what is the concentration of hydronium ion in the solution?
Nicotine, C 10 H 14 N 2 , is a base that will accept two protons ( K b1 = 7 10 −7 , K b2 = 1.4 10 −11 ). What is the concentration of each species present in a 0.050- M solution of nicotine?
The pH of a 0.23- M solution of HF is 1.92. Determine K a for HF from these data.
The pH of a 0.15- M solution of is 1.43. Determine K a for from these data.
The pH of a 0.10-
M
solution of caffeine is 11.70. Determine
K
b
for caffeine from these data:
The pH of a solution of household ammonia, a 0.950 M solution of NH 3, is 11.612. Determine K b for NH 3 from these data.
Determine whether aqueous solutions of the following salts are acidic, basic, or neutral:
(a) Al(NO 3 ) 3
(b) RbI
(c) KHCO 2
(d) CH 3 NH 3 Br
Determine whether aqueous solutions of the following salts are acidic, basic, or neutral:
(a) FeCl 3
(b) K 2 CO 3
(c) NH 4 Br
(d) KClO 4
Novocaine, C 13 H 21 O 2 N 2 Cl, is the salt of the base procaine and hydrochloric acid. The ionization constant for procaine is 7 10 −6 . Is a solution of novocaine acidic or basic? What are [H 3 O + ], [OH − ], and pH of a 2.0% solution by mass of novocaine, assuming that the density of the solution is 1.0 g/mL.
Which of the following concentrations would be practically equal in a calculation of the equilibrium concentrations in a 0.134- M solution of H 2 CO 3 , a diprotic acid: [OH − ], [H 2 CO 3 ], No calculations are needed to answer this question.
Calculate the concentration of each species present in a 0.050- M solution of H 2 S.
Calculate the concentration of each species present in a 0.010-
M
solution of phthalic acid, C
6
H
4
(CO
2
H)
2
.
Salicylic acid, HOC 6 H 4 CO 2 H, and its derivatives have been used as pain relievers for a long time. Salicylic acid occurs in small amounts in the leaves, bark, and roots of some vegetation (most notably historically in the bark of the willow tree). Extracts of these plants have been used as medications for centuries. The acid was first isolated in the laboratory in 1838.
(a) Both functional groups of salicylic acid ionize in water, with K a = 1.0 10 −3 for the—CO 2 H group and 4.2 10 −13 for the −OH group. What is the pH of a saturated solution of the acid (solubility = 1.8 g/L).
(b) Aspirin was discovered as a result of efforts to produce a derivative of salicylic acid that would not be irritating to the stomach lining. Aspirin is acetylsalicylic acid, CH 3 CO 2 C 6 H 4 CO 2 H. The −CO 2 H functional group is still present, but its acidity is reduced, K a = 3.0 10 −4 . What is the pH of a solution of aspirin with the same concentration as a saturated solution of salicylic acid (See Part a).
The ion HTe − is an amphiprotic species; it can act as either an acid or a base.
(a) What is K a for the acid reaction of HTe − with H 2 O?
(b) What is K b for the reaction in which HTe − functions as a base in water?
(c) Demonstrate whether or not the second ionization of H 2 Te can be neglected in the calculation of [HTe − ] in a 0.10 M solution of H 2 Te.
Explain why a buffer can be prepared from a mixture of NH 4 Cl and NaOH but not from NH 3 and NaOH.
Explain why the pH does not change significantly when a small amount of an acid or a base is added to a solution that contains equal amounts of the acid H 3 PO 4 and a salt of its conjugate base NaH 2 PO 4 .
Explain why the pH does not change significantly when a small amount of an acid or a base is added to a solution that contains equal amounts of the base NH 3 and a salt of its conjugate acid NH 4 Cl.
What is [H
3
O
+
] in a solution of 0.25
M
CH
3
CO
2
H and 0.030
M
NaCH
3
CO
2
?
What is [H
3
O
+
] in a solution of 0.075
M
HNO
2
and 0.030
M
NaNO
2
?
What is [OH
−
] in a solution of 0.125
M
CH
3
NH
2
and 0.130
M
CH
3
NH
3
Cl?
What is [OH
−
] in a solution of 1.25
M
NH
3
and 0.78
M
NH
4
NO
3
?
What is the effect on the concentration of acetic acid, hydronium ion, and acetate ion when the following are added to an acidic buffer solution of equal concentrations of acetic acid and sodium acetate:
(a) HCl
(b) KCH 3 CO 2
(c) NaCl
(d) KOH
(e) CH 3 CO 2 H
What is the effect on the concentration of ammonia, hydroxide ion, and ammonium ion when the following are added to a basic buffer solution of equal concentrations of ammonia and ammonium nitrate:
(a) KI
(b) NH 3
(c) HI
(d) NaOH
(e) NH 4 Cl
What will be the pH of a buffer solution prepared from 0.20 mol NH 3 , 0.40 mol NH 4 NO 3 , and just enough water to give 1.00 L of solution?
Calculate the pH of a buffer solution prepared from 0.155 mol of phosphoric acid, 0.250 mole of KH 2 PO 4 , and enough water to make 0.500 L of solution.
How much solid NaCH 3 CO 2 •3H 2 O must be added to 0.300 L of a 0.50- M acetic acid solution to give a buffer with a pH of 5.00? (Hint: Assume a negligible change in volume as the solid is added.)
What mass of NH 4 Cl must be added to 0.750 L of a 0.100- M solution of NH 3 to give a buffer solution with a pH of 9.26? (Hint: Assume a negligible change in volume as the solid is added.)
A buffer solution is prepared from equal volumes of 0.200 M acetic acid and 0.600 M sodium acetate. Use 1.80 10 −5 as K a for acetic acid.
(a) What is the pH of the solution?
(b) Is the solution acidic or basic?
(c) What is the pH of a solution that results when 3.00 mL of 0.034 M HCl is added to 0.200 L of the original buffer?
A 5.36–g sample of NH 4 Cl was added to 25.0 mL of 1.00 M NaOH and the resulting solution
diluted to 0.100 L.
(a) What is the pH of this buffer solution?
(b) Is the solution acidic or basic?
(c) What is the pH of a solution that results when 3.00 mL of 0.034 M HCl is added to the solution?
Explain how to choose the appropriate acid-base indicator for the titration of a weak base with a strong acid.
Explain why an acid-base indicator changes color over a range of pH values rather than at a specific pH.
Calculate the pH at the following points in a titration of 40 mL (0.040 L) of 0.100 M barbituric acid ( K a = 9.8 10 −5 ) with 0.100 M KOH.
(a) no KOH added
(b) 20 mL of KOH solution added
(c) 39 mL of KOH solution added
(d) 40 mL of KOH solution added
(e) 41 mL of KOH solution added
The indicator dinitrophenol is an acid with a K a of 1.1 10 −4 . In a 1.0 10 −4 - M solution, it is colorless in acid and yellow in base. Calculate the pH range over which it goes from 10% ionized (colorless) to 90% ionized (yellow).


4.3: Impulse
Learning Objectives
- Define impulse.
- Describe effects of impulses in everyday life.
- Calculate average force and impulse given mass, velocity, and time.
The effect of a force on an object depends on how long it acts, as well as how great the force is. In previous examples, a very large force acting for a short time had a great effect on the momentum of the tennis ball. A small force could cause the same change in momentum , but it would have to act for a much longer time. For example, if the ball were thrown upward, the gravitational force (which is much smaller than the tennis racquet’s force) would eventually reverse the momentum of the ball. Quantitatively, the effect we are talking about is the change in momentum \(\Delta \boldsymbol{p}\).
By rearranging the equation \(\boldsymbol{F}_{\mathrm{net}}=\frac{\Delta \boldsymbol{p}}{\Delta t}\) to be
\[\Delta \boldsymbol{p}=\boldsymbol{F}_{\mathrm{net}} \Delta t, \nonumber \]
we can see how the change in momentum equals the average net external force multiplied by the time this force acts. The quantity \(\boldsymbol{F}_{\text {net }} \Delta t\), force times duration, is given the name impulse . Net impulse on an object gives its change in momentum.
IMPULSE: CHANGE IN MOMENTUM
Change in momentum equals the average net external force multiplied by the time this force acts: \(\Delta \boldsymbol{p}=\boldsymbol{F}_{\mathrm{net}} \Delta t\).
The quantity \(\boldsymbol{F}_{\text {net }} \Delta t\) is given the name impulse.
There are many ways in which an understanding of impulse can save lives, or at least limbs. The dashboard padding in a car, and certainly the airbags, allow the net force on the occupants in the car to act over a much longer time when there is a sudden stop. The momentum change is the same for an occupant, whether an air bag is deployed or not, but the force (to bring the occupant to a stop) will be much less if it acts over a larger time. Cars today have many plastic components. One advantage of plastics is their lighter weight, which results in better gas mileage. Another advantage is that a car will crumple in a collision, especially in the event of a head-on collision. A longer collision time means the force on the car will be less. Deaths during car races decreased dramatically when the rigid frames of racing cars were replaced with parts that could crumple or collapse in the event of an accident.
Bones in a body will fracture if the force on them is too large. If you jump onto the floor from a table, the force on your legs can be immense if you land stiff-legged on a hard surface. Rolling on the ground after jumping from the table, or landing with a parachute, extends the time over which the force (on you from the ground) acts.
Our definition of impulse includes an assumption that the force is constant over the time interval \(\Delta t\). Forces are usually not constant . Forces vary considerably even during the brief time intervals considered. It is, however, possible to find an average effective force \(F_{\mathrm{eff}}\) that produces the same result as the corresponding time-varying force. Figure \(\PageIndex{1}\) shows a graph of what an actual force looks like as a function of time for a ball bouncing off the floor. The area under the curve has units of momentum and is equal to the impulse or change in momentum between times \(t_{1}\) and \(t_{2}\). That area is equal to the area inside the rectangle bounded by \(F_{\mathrm{eff}}\), \(t_{1}\), and \(t_{2}\). Thus the impulses and their effects are the same for both the actual and effective forces.
MAKING CONNECTIONS: TAKE-HOME INVESTIGATION—HAND MOVEMENT AND IMPULSE
Try catching a ball while “giving” with the ball, pulling your hands toward your body.  Hit water in a tub with your full palm. After the water has settled, hit the water again by diving your hand with your fingers first into the water. (Your full palm represents a swimmer doing a belly flop and your diving hand represents a swimmer doing a dive.) Explain what happens in each case and why. Which orientations would you advise people to avoid and why?
MAKING CONNECTIONS: CONSTANT FORCE AND CONSTANT ACCELERATION
The assumption of a constant force in the definition of impulse is analogous to the assumption of a constant acceleration in kinematics. In both cases, nature is adequately described without the use of calculus.
Section Summary
-
Impulse is force times duration, and the net impulse, equal to the net external force multiplied by duration, gives the change in momentum:
\[\Delta \boldsymbol{p}=\boldsymbol{F}_{\text {net }} \Delta t. \nonumber\]
- For non-constant net external force, you need to use time-averaged net external force to calculate the impulse over the duration.
Glossary
- change in momentum
- the difference between the final and initial momentum; the mass times the change in velocity
- impulse
- the average net external force times the time it acts; equal to the change in momentum