# Measure force of impact

1. Nov 21, 2009

### joanneneil

Hello,

My 7th grade son has selected a science project which requires to find the impact of two toy cars crashing as part of the larger project. To keep it simple, he will keep one car fixed (i.e. not moving) and the other car crashing into the fixed car at varying speed. I have suggested to use F=ma equation to calculate the force of impact - where mass can be found by weighing the toy car and the acceleration or speed can be found by measuring the distance covered and time it takes to cover the distance by a stop watch assuming the speed is constant. Am I in the right track suggesting this? Is there any other way (simpler) to measure or calculate the impact?

Thanks and Regards
Joanne

2. Nov 22, 2009

### tiny-tim

Welcome to PF!

Hello Joanne! Welcome to PF!

F = ma is really F = d(mv)/dt, where mv is the momentum.

In that form, you can see that it's really the momentum that matters, and in a crash, we use impulse rather than force, where impulse is force times (a very short) time …

impulse = ∆(mv), ie impulse = change in momentum.

(you don't really need to know the actual force or acceleration, unless you're examining the crumpling of the cars, or the g-forces on the occupants, neither of which, I suspect, are possible with toy cars! )​

3. Nov 22, 2009

### A.T.

What exactly is he supposed find out about the impact? Computing the peak force is far from simple in this case.

If you assume the speed is constant, the acceleration and force are both zero. Not a very realistic assumption about the impact.

"The impact" is not a physical quantity you can measure or calculate. The peak force during the impact is proportional to the peak acceleration, but measuring the peak acceleration during impact is more difficult than measuring the peak force itself.

4. Nov 22, 2009

### xxChrisxx

Before we go any further with this, you appear to be making a critical and common error OP.

The way the first post reads is that:

You think that the force of an impact depends on how much it accelerated before impact. The force involved depends on how fast it slows down.

So if you had two real cars, both the same and both accelerating. One hits a brick wall, the other hits a large soft rubber wall. The one that hits the brick wall has to get rid of its momentum really quickly, (it goes from whatever speed to 0 very quickly) the forces are high. The soft rubber wall allows the car to decelerate slower, meaning lower forces. This is explained by impulse and momentum, outlined by tiny tim.

If that's not what you meant then please disregard the above.

Other than that you are on the right track, trying to measure impact time can be tricky though as they are so short.

Another method is to assume an elastic collison and use conservation of momentum, and conservation of kinetic energy. This will not be strictly true but stops the problems of trying to measure the coefficient of restitution of a toy car. Which is undoubtedly WAAAAAY beyond what you would be wanting for a year 7 project.