# Impact force after collision

1. Feb 17, 2014

### get_physical

1. The problem statement, all variables and given/known data
1200kg car moving at 9.72m/s runs into and sticks to a parked 1000kg car. The collision takes 0.15s and after the crash the cars wheels lock and they skid to a stop on u=0.40 road. What is the impact force on both cars?

2. Relevant equations
J=F*t=m*dv

3. The attempt at a solution
I've found the distance it takes for it to stop (3.68m) and also the speed after the crash (5.3m/s).

I used the momentum to divide by the time (0.15s) to get the impact force, but the answer is wrong.

2. Feb 17, 2014

### jackarms

Did you make sure to use the change in momentum? Otherwise, your calculations for the speed after the crash and distance may be off.

3. Feb 18, 2014

### get_physical

The calculations for the speed after the crash and the distance are correct. But I just can't find the impact force.

4. Feb 18, 2014

### Staff: Mentor

Show your arithmetic. Maybe you are making a mistake.

5. Feb 18, 2014

### lendav_rott

What does it mean that the collision takes 0.15 seconds? That deformation takes 0.15 seconds after which the cars start skidding with the remaining energy?

Oh Ok, I think I understand. Think about kinetic energies. If you knew the amount of energy and the time it was being applied, you can find out the force affecting the target mass.
Before the collision, the "speeding" car as a kinetic energy of N. After the collision, the 2-car-mass has a kinetic energy of M. M is dissipated by friction over time. What happened to the N-M , though?

Last edited: Feb 18, 2014
6. Feb 18, 2014

### BvU

Dear get,

All this could have been short-circuited if you had shown why F = 1200 kg * (9.72 - 5.3)m/s / 0.15 s did not yield the right answer. Someone would have told you to check your calculation against 1000 kg * 5.3 m/s / 0.15 s

7. Feb 18, 2014

### get_physical

Thank you. I used F= 2200 (0 - 5.3m/s) /0.15
Why do we only use 1200kg car and why do we not use 1000kg car?

8. Feb 18, 2014

### Staff: Mentor

To find the force on a car, you must analyze the change in momentum of that car. Not both together! You can use either car.