# Inelastic Collisions

1. Jun 25, 2008

### ace123

A 920-kg sports car collides into a rear end of a 2300 kg SUV stopped at a red light. The bumpers lock, the brakes are locked, and the two cars skid foward 2.8 m before stopping. The police officer, knowing that the coefficient of kinetic friction between tires and road is .80 , calculates the speed of the sports car at impact. What was that speed?

This is all that I did before I got stuck.

I set up the conservation of momentum :920 kg * V$$_{1}$$= 3220 * V$$_2{}$$

Then I did F$$\Delta$$t= $$\Delta$$P

I found the force to be 25244= 3220 V$$_{2}$$ / $$\Delta$$t

Now is where I get stuck. I think I did something wrong because I'm pretty sure I need to do v/t =d.

Also not sure if I should use kinetic energy in this one since it's not conserved.

Any help will be great. Thanks as always.

On a side note: Isn't it hilarious how the officer knows the kinetic friction and calculates the speed.

2. Jun 25, 2008

For this problem you need to work backwards. You have equations of kinematics, from which you can calculate the inital speed of the two interlocked cars (remember that for any force, including friction, F=m*a).

After that you can used conservation of momentum (the fact that m1v1=m2v2) to calculate the initial speed of the moving car.

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