- #1

Jimmy87

- 680

- 15

I was watching a documentary on car crashes and how designs are continually being optimised to keep passengers safe. They were saying how many ‘g’s’ certain crashes were based on crumple zones, seatbelt stretch etc. I was interested to do some number crunching to see how big the decelerations are and came across this tool:

https://www.omnicalculator.com/physics/car-crash-force

If you put in 80kg and 50mph it generates 127.4g of deceleration. The kinetic energy of a 80kg person at 50mph person is 19,984J which is correct if you do the calculations. If you tick the wearing a seatbelt option it says the distance it stops you over is 16cm. If you divide the kinetic energy by 0.016m to get the force you get 124,900N. If you use F=ma to find the acceleration (124,900/80) you get an acceleration of 159g.

However if you do the change in speed (50mph is 22.35m/s) divided by the collision time they give of 0.018s (18ms) you get 124g. Why are the two ways of finding the deceleration 24g out? Also how would they have calculated the collision time as surely there are many factors that affect it so where do they get 18ms from?

Thanks