# Homework Help: Car wreck problem

1. Nov 22, 2009

### jared13

1. The problem statement, all variables and given/known data
Fresh from CSI training, you are sent to the scene of an accident. A 1,600kg Jeep Cherokee is locked in a crumpled pile of metal with what used to be a 900kg Chevy convertible. A witness tells you that the convertible was waiting at the light when the Jeep had hit its brakes and slid into the convertible, whereupon the convertible was crushed and the Jeep and convertible slid locked together until they came to rest. The witness also tells you that that at the time of the accident, the road surface was dry, just as it is when you arrive. After making some measurements, you determine that the coefficient of sliding (kinetic friction) beween the vehicle tires and the dry roadsurface is about .45. You also measure skid marks 23m in length on the road behind where the convertible was hit, and another 13m of skidmarks clearly form the jeep and convertible as they slid through the traffic light locked together. You check the road signs and see that the speed limit on that section of road is 45mph. Was the jeep speeding before it hit its brakes? (If so by how much?)

2. Relevant equations

F=ma
x=x0+v0xt+1/2axt^2
v=v0x+axt
v^2x=v0x^2+2axx

3. The attempt at a solution

Please help me I really dont know where to begin. If I had to take a guess I would say the coefficent of friction and the length of the skid marks has something to do with being able to judge how fast the car was going, but I do not not how they are linked.

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Nov 22, 2009

### mgb_phys

Conservation of energy.
At the start you have kinetic energy of the jeep.
All this is converted into friction (if you assume the bang lost no energy!!!)

Remember energy is force * distance,
How o you work out frictional force?