The Khan Academy. Surely you have heard of it – right? Well, just in case, let me summarize. Sal Khan – made some video tutorials on all sorts of stuff. Popular media hails Khan as the chosen one of education. …

Dot Physics

Video Breakdown: Khan and Kinematics


So my quick take – Khan Academy is a textbook video. Are textbooks new? No. Are they the best thing for students? No. Should we ban textbooks in all forms? I don’t think so.

What about other aspects of Khan Academy videos? How are they based on content? I happened to take a look at Khan’s 3 videos on intro to motion. Let me break down what I found.

First, the video. Here is part I.

And part II and part III. Ok. Here are my comment.

Mechanics is pretty intuitive

No, it isn’t. Mechanics is like the opposite of intuitive. If it were intuitive, wouldn’t it be a pretty simple subject? In fact, the problem with mechanics is that the models we build go against common ideas about force and motion. Aristotle got all this stuff wrong, but people agreed with him.

Distance equals rate times time

You Tube   Introduction to Motion

I hate this definition of velocity or whatever you want to call it. Sure, lots of middle school textbooks say this -but that doesn’t mean it is a good idea. In part III, Khan sort of gives the correct definition of velocity but why not start off on the right foot. I like this definition of average velocity (in the x-direction):

La te xi t 1

The most important thing about velocity is that it relates to the CHANGE in position and the CHANGE in time. Why is this so important to emphasize? Mostly because this is something students have trouble with – especially when they move to acceleration.

Velocity vs. Speed and Vector vs. Scalar

Khan briefly discusses the difference between speed and velocity. He claims that speed is a scalar quantity and velocity is a vector. Ok, not too bad, but I think there is more. The commonly used definition says that speed is how far you go (distance) over how long it took you. Velocity is change in displacement over change in time.

Take an example. Suppose you have a car going around a race track. What is the average velocity for one trip around? Well, it starts and ends at the same place such that the change in displacement is zero. The average velocity is zero. Of course, the speed is not zero.

Khan also states that velocity can be negative because it is a vector. Let me just add that it can have components that are negative – the whole vector is not negative.

If you manipulate the units, you can get the right answer

Units are clearly important. However, you can’t only use them to find your answer. If use Khan’s two equations:


It would seem like the next step should say:

La te xi t 1 1

Which does have the correct units, but is wrong. Khan later goes on to show how to make this work, but still wouldn’t it be better to start off on the right foot? (or the left foot if you are left footed).

Projectile Problems

If I had to classify all the problems in a first semester of physics, I would probably use categories like this:

  • Kinematics
  • Momentum Principle
  • Work Energy Principle

Students would rather classify like this:

  • Block on a plane
  • Gravity problems
  • Problems with ropes and tensions
  • And yes, projectile problems

There was some research paper that looked at how experts vs. novices classified problems. If I recall correctly, it was interesting. For an expert, you could have a problem with gravity. It could be a work-energy problem or a momentum principle problem. The context does not define the type. Khan seems to be classifying like a novice.

Acceleration due to gravity

Like many physicists, I hate this term. If I put a book on a table, there is a gravitational force on it (with a magnitude of mg). The g is clearly important – but the book is not accelerating. What should we call it? We should call it as many people do the local gravitational field magnitude or maybe just the gravitational constant. I even like to give it the units of N/kg – which is equivalent to m/s2.

Calling it the “acceleration due gravity” isn’t so much wrong as it is annoying.

Dropping a penny off the Empire State Building

Why would you start with an example like this? Khan claims this is a fairly simple example – well, not really that realistic is it? If I drop a penny from that high, it have the effects of air drag on it and make it not so simple. He might as well have found the acceleration of a unicorn jumping through a rainbow because that is just as valid.

Maybe I am being picky

Ok. Sure there are some mistakes. Who doesn’t make mistakes. Everyone knows I make mistakes. In this case, I think the frequency of mistakes is a little bit higher than it should be. Especially considering how much attention these videos have. The point is – these videos are like textbooks. Mediocre textbooks at best (at least based on these three videos).

Why is Khan Academy so popular?

Listening to Sal Khan, it reminds me of these SAT/ACT prep courses. The goal of these courses is to give you the ‘recipes’ you need to wkr the problems and get a higher score. The goal is not to give you a better understanding of the material. Khan seems to have the same approach “let me show you how to do it”.

Bonus Khan

This is super old, but super funny. You of course know of Star Trek the Wrath of Khan, right?


The part where Kirk yells: KHAAAAAAAN! is sort of an internet meme and used quite often. The question is – how many A’s do you put in there? Here is a plot.


I left off hits with just one “A” because those could be Sal Kahn, Wrath of Khan or Genghis Khan. Also, I left off the results with 2 A’s – those could just be a typo. Notice the nice spike for 7 A’s? It seems like common opinion says that is how many A’s you should put in KHAAAAAAAN. Although, if you put 62 As in your KHAAAAAN, you are not alone.

Rhett Allain


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