posted by kago .
A 0.90 × 10^3 kg sports car collides into the rear end of a 2.4 × 10^3 kg SUV stopped at a red light. The bumpers lock, the brakes are locked, and the two cars skid forward 4.6 m before stopping. The police officer, estimating the coefficient of kinetic friction between tires and road to be 0.40, calculates the speed of the sports car at impact. What was that speed?
The sliding distance can be used to compute the kinetic energy of the joined cars.
(friction force)*X = Kinetic energy at the beginning of skid.
From the KE and masses, get the initial momentum of the joined cars.
Once you have the momentum of the joined cars, you know that it equals the momentum of the sports car before collision.