Question

How fast must a car be travelling to generate 1G (gravity) of force on itself and it's occupants when stopped, as if hitting a wall?

Asked by: Kevin Watton

Answer

In fact, a car's velocity and the number of 'g''s you would experience when the car hits a wall are completely different concepts.

A 'g' is a measure of acceleration, the rate of change of velocity. To know how much acceleration the car experiences when stopping you have to know something about the structure of the car and what its hitting as well as its initial speed. Look at it this way, if you drop an egg out the window onto the pavement it will break. This is because the egg stopped very quickly when it reached the pavement, therefore its velocity was changing very rapidly and there was consequently a large acceleration. If you wrapped the same egg in bubble wrap and dropped it out of the window, it would have about the same energy when it hit, but the acceleration it experienced during the collision would be a lot less because it has some cushioning. This is precisely the reason why car manufacturers put 'crumple zones' in cars: you experience less 'g force' during a crash because the car can collapse and take longer to stop.

Answered by: Mike Rust, Undergraduate Physics, Harvey Mudd College

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'The difference between what the most and the least learned people know is inexpressibly trivial in relation to that which is unknown.'

Albert Einstein
(1879-1955)