Originally Posted by
Khendraja'aro
Okay, some small calculations to bring my point across.
Let's take a car travelling at 90 Km/h, that's 25 m/s. Let's have that car have a crumple zone of 1 meter or an equivalent thereof. Which means that you go from 90 Km/h to 0 Km/h over the length of 1 meter.
For that, I'll need two formulas: v = a * t and s = a/2 * t^2
v = velocity (90 Km/h or 25 m/s), s = length (1 m), t = time (unknown), a = deceleration (unknown).
Rearranging them yields:
s = a/2 * (v/a)^2 <=> a = 0.5 * v^2/s
Using the values listed above, we arrive at a deceleration of a = 312.5 m/s^2 which is about 32 times the gravitational acceleration or 32 "gees". That is indeed survivable. Broken bones, surely, but survivable.
Let's assume now that you did not wear your seat belt, thus being catapulted out of it, resulting in a deceleration a the windshield. For shits'n'giggles, let's also assume that the windshield is unbreakable. You now have a deceleration length of mere centimeters, i.e. your skin. Let's be generous and make that 3 centimeters total.
This results in: a = 10416 m/s^2. Which is about 1061 g. Okay, that would be rather deadly. Thankfully, the windshield is not unbreakable.
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Same stuff for riding a bike. Let's assume you managed to brake somewhat and were hurled over. Thus arriving with your head on the street at, say, 20 Km/h.
With your helmet, you have a deceleration equivalent length of, say, 6 centimeters.
This gives us: a = 252m/s^2 or 26 g
Without a helmet, we use the 3 centimeters again and arrive at a whoppin' 52 gees.
And I'm pretty sure that those 3 centimeters are very generous.