Question

You’re an accident investigator at a scene where a drunk driver in a 2300 kg car has plowed into a 1200 kg parked car with its brake set. You measure skid marks showing that the combined wreckage moved 24 m before stopping, and you determine a frictional coefficient of 0.68.

What do you report for the drunk driver’s speed just before the collision?

Answer 1

Mass of the driving car, and mass of the parked car,

Mass of the combined wreckage after collision,

Frictional force acting on the cars,

Work done by friction, W= Frictional force x Distance moved =

Due to friction the wreckage decelerates and the kinetic energy of the wreckage is lost as work done by friction.

If be the velocity of the wreckage just after collision then,

Kinetic energy of the wreckage = Work done by friction

Let be the speed of the driving car. Then the conservation of momentum gives

Momentum of the driving car before collision = Momentum of wreckage after collision

Therefore the drunk driver's speed just before collision is 27.2 m/s