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MOTION · B1 · 001 · Impulse

Your car is designed to destroy itself. On purpose.

Impulse: force × time = mass × change in speed. In a crash your change in speed is fixed, so the only thing engineers can change is the time. More time, less force.

Try itSame stop, more time

A person stops from crash speed. Speed and mass set the impulse: the shaded area on the graph. Stopping time only changes its shape: a longer stop makes it flatter, so the force drops.

Average force–
Like a weight of–
Body feels–
Stopping distance–

NumbersHow big is it?

Sim forces are averages over the stop (force = mass × change in speed ÷ time), with an even slowdown. Real crash forces peak higher than the average.

≈ 10 t

Average force on a 75 kg person stopping from 50 km/h in 10 ms: about 104 kN, the weight of about 10.6 tonnes. In 100 ms it is about 1 tonne.

62 cm vs 7 cm

How far the driver's footwell was pushed in during the IIHS 40 mph (64 km/h) offset test: 1959 Chevrolet Bel Air vs 2009 Chevrolet Malibu.

Source ↗
85 g vs 41 g

Peak chest acceleration of the driver dummy in the same test. The old cabin collapsed; the new one held while its front crushed.

Source ↗

HowWhat happens, step by step

  1. Your change in speed is fixed.

    At 50 km/h into a wall you end at zero. Nothing in the car can change that part.

  2. Engineers stretch the time instead.

    The front folds like an accordion. Every centimetre it crushes adds milliseconds to your stop.

  3. The cabin stays rigid.

    The strong cage around you keeps its shape, so nothing folds into the space where you sit.

Then the belt and airbag stretch your own stop even more: the belt gives a little, the airbag deflates as you hit it.

Check3 quick questions

Bonus

Formula 1 cars use the same trick: the nose is built to crush and soak up the crash, while the cockpit stays rigid. Stretch the stop in the sim and watch the force fall.