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Comment by oldandtired | Hacker News Reader

Here in Victoria (Australia), we commonly see road signs stating that "speed kills" whereas the reality is that it is the jerk that kills.

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No, it's the acceleration. That's what produces the forces. But there is usually a correlation between jerk and acceleration in a collision, given a certain amount of energy (speed^2)

I didn't think my play on words would cause such controversy about acceleration and jerk. Mayhaps I should not be making "Dad jokes" here.

WBRTC in India used to have noticeboards and buses painted with “Safe Drive Save Life” and “Save Drive Safe Life”

I’m not sure anyone noticed the difference between the two

Here are my interpretations:

* If you drive safely, then you can save someone's life by avoiding a crash.

* If you save a driving trip (and instead walk/bike/transit/stay-home), you will have a life of safety.

As Jeremy Clarkson put it "Speed never killed anybody. Suddenly becoming stationary? That's what gets you."

I have a pair of matching Nike socks, where one of them said that, and the other one replies “fast is faster”

If you want to be pedantic, you could argue that it is the collision that kills.

Is it not life that kills?

I'm pretty sure it's time.

It isn't, though. It's the jerk.

The difference between falling from a height and landing on a trampoline, and landing on concrete from the same height, is that the trampoline smoothly accelerates you to a halt once you collide with it. The concrete does so much more rapidly: that's jerk. Both of these are collisions with the same amount of force behind them.

No, it's the acceleration. The forces are absolutely different between your two examples. The energy is the same, but in one case the energy is dissipated over a longer time, requiring less acceleration and thus force. Jerk and acceleration will tend to be correlated in collisions, but you can construct cases with very high jerk and low acceleration which are just fine and cases with very high acceleration and low jerk which are not at all safe, though the latter case will generally require a lot more energy (fighter jets and rockets, for example).

Airbags are the most important safety feature in cars. They don't change the rate of acceleration: the car comes to a halt at the same speed, therefore, the person inside does as well.

What they affect is jerk.

No, they absolutely do change the rate at which the person inside stops: most importantly they affect the amount of time it takes for their head to stop moving, by starting to decelerate it before it hits the dashboard (the seatbelts also contribute to this).

In other words they change the rate of change of the acceleration. We call that derivative jerk. Jerk is responsible for the shockwave which would otherwise rattle the brain around, it's also the chief factor in producing whiplash.

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