what was that speed?

Hi
I need your help about this problem.  Would you help me?


A 900 kg Geo collides into the rear end of a 2000 kg Lexus stopped at a red light.

The bumpers lock, the brakes are locked, and the two cars skid forward 4.5 m

before stopping on the horizontal road. The police officer, knowing that the

coefficient of kinetic friction between tires and road is 0.40, calculates the speed of

the Geo at impact. What was that speed?

Thank you

I have one more question. Please help me

A person rides in an elevator that is falling but is slowing down as it falls.

Dose it true; that the force of the elevator floor is doing positive work on the person!??

Thank you again