Monthly Archives: October 2012

Surviving acceleration

How fast can you accelerate, or decelerate, and live to tell the tale?

In this context, acceleration and deceleration are usually measured in ‘G’s, multiples of the acceleration due to gravity. For example, if you crashed a car travelling at 70 miles per hour into a wall, and it took you one second to come to a stop this would be a deceleration of 35.8 metres per second per second, which is equivalent to an deceleration of 3.65 G. If the person in the car had a weight of 1000 newtons (≈100 kg) they would feel a force pushing them forwards against their seatbelt of 3650 N.

G-forces on the human body are described in two ways*: Gx which is along an axis running horizontally through the chest at a right angle and Gz which is along an axis running vertically downwards through the head and feet. A positive Gx is described as “eyeballs in” and a negative Gx as “eyeballs out”; a positive Gz pushes blood towards the feet and a negative Gz pushes blood towards the head.

The human body responds differently to acceleration in different directions.

For example: a human being can survive an “eyeballs in” 5G acceleration for about 1500 seconds, but an “eyeballs out” 5G acceleration for only half of that. Moving vertically, with blood towards the feet, a 5G acceleration can only last for 350 seconds before death occurs; but with blood towards the head for only about 8 seconds.

It seems that the human body is least sensitive to “eyeballs in” and “eyeballs out” accelerations, which makes sense when considering that human beings are far more prone to experience these accelerations than others. It’s easier to survive blood rushing to the feet than it is to survive blood rushing to the head, as increased blood pressure in the head can cause blood vessels in the brain to burst.

* It seems that very little work has been done on how the body responds to sideways accelerations.

How are mushroom clouds formed?

Mushroom clouds (perhaps more properly known as pyrocumulus clouds) are traditionally associated with nuclear explosions, but any sufficiently large explosion (for example, a volcanic eruption) will create a mushroom cloud.

The mushroom cloud resulting from the Priscilla test of Operation Plumbbob.

When a large explosion occurs a cloud of very hot gas is created. This hot gas, being less dense than the surrounding air, rises rapidly upwards. As this cloud of hot gas rises it pushes against the air above it and this air resistance causes the top layer to move sideways whilst the hotter gas below continues rising upwards, creating a swirling doughnut-shaped vortex (in the photograph above a very hot “filament” is visible at the centre of this vortex). As the “cap” rises this swirling vortex pulls in cooler air from ground level, creating the “stalk” on which the cap sits.

The formation of a mushroom cloud during the Tumbler-Snapper series of nuclear tests.

The shape of a mushroom cloud is the result of a Rayleigh-Taylor instability at the interface between the hot less-dense and cold more-dense air. These instabilities occur in a number of different situations, and can be easily demonstrated at home by dropping coloured oil into water, creating tiny upside-down mushroom clouds as shown below in photographs by James Riordon of AIP.

The simulated formation of a Rayleigh-Taylor instability.











The Trivers-Willard hypothesis

The Trivers-Willard hypothesis states that when conditions are good, parents have more male offspring; and when conditions are poor, parents have more female offspring. The thinking behind this is that in favourable conditions males will be able to mate with many females before they die and have a greater chance of passing on their genes; and when conditions are poor males will not be able to mate with as many females and are more likely to be out-competed, and therefore a female will have more chance of passing on genetic material than any particular male.

The Trivers-Willard hypothesis seems to hold true for human beings. In a study of the Forbes Billionaires List it was found* that the children of billionaires were 60% male, and if only male billionaires were considered then this percentage rose to 65%. The effect was the same whether the billionaires were self-made or had inherited their fortunes, suggesting that if there was a biological reason for success in business it was not relevant in selecting the sex of offspring.

It is uncertain what causes the Trivers-Willard effect, but a 2001 paper suggested† that “condition” was linked to the availability of food and of glucose in this food, and that the presence of elevated levels of glucose in the mother’s blood favours the survival of male blastocysts. This has led to the idea of the “Trivers-Willard Diet“, designed to enable parents to select the sex of their offspring.

* Elissa Cameron and Fredrik Dalerum, “A Trivers-Willard Effect in Contemporary Humans: Male-Biased Sex Ratios among Billionaires”, PLoS ONE 4(1) (2009): e4195. DOI: 10.1371/journal.pone.0004195.

† Melissa Larson et al, “Sexual dimorphism among bovine embryos in their ability to make the transition to expanded blastocyst and in the expression of the signaling molecule IFN-τ”, Proc Natl Acad Sci 98(17) (2001): 9677-9682. DOI: 10.1073/pnas.171305398.

Nuclear art forgery



