Question

a 1200kg car traveling initially qith a speeed of 25.0 m/s in an easterly direction crashes into the rear end of a 9,000kg truck moving in the same direction at 20.0 m/s. the velocity of the car right after the collision is 18.0 m/s to the east. what is the celocity of the truck right after the collision?

Answer 1

Since this is NOT an elastic collision, only momentum is conserved. You only need to conside momentum on east-west (x) direction.

Mcar*25.0 + Mtruck*20.0 = Mcar*18.0 + Mtruck*V
V is the velocity you want.
210,000 = 21,600 + 9000 V
V = 20.93 s