# Collision question

1. Jul 25, 2007

### Steven60

Suppose you drive your car into a wall at the rate of 50 mph. Would the damage to the car depend on whether you were accelerating to 50 mph verus a constant speed of 50 mph? What if there was no running engine on impact?
Thanks

2. Jul 25, 2007

### mgb_phys

It depends on how quickly you slow down when you hit the wall ,
which depends on 1, the speed when you hit the wall, 2, the strength of the wall and most importantly 3, the engineering of how your car deforms.

Hwo you were accelerating at that moment or the running engine don't make any difference - unless the car or wall squashes slowly enough that you continue accelerating as you squash into it.

3. Jul 25, 2007

### mjsd

(thinking in idealised terms) the damage would be bigger if you continue to accelerate because the wall wants the front end of car to stop but the acceleration is trying to keep pushing the car forward causing the car to squash even more. (but then by that time your engine is probably gone) now, you need to clarify your question a bit. when does the car actually stop accelerating and start "decelerating", the quicker (over shorter distance) it goes from 50 to 0, the bigger the impact/damage. as you can imagine if the wall breaks the car will not stop at the wall but a bit further down and as a result the stopping distance becomes longer, and hence in general less damage by comparsion is done on the car. just like an F1 hitting the tire wall, if the tires are as hard as rocks, it will not do any good. the fact that they are effective b/c they are soft and they breaks on impact.
so, stopping distance and time it takes to decelerate to 0 mph are the keys here. BUT having said that , we have assumed a lot here. and the real world is not that idealistic.