Colliding Freight Cars: Kinetic Energy Transformed

[CanadianEh]

I am totally lost with this one... please explain if you know how to do it.


Two 5000-kg freight cars roll toward one another (one at 1 m/s and the other at 2 m/s) on a level track. Thus before they collide one car's kinetic energy is 2500 J and the other's is 10,000 J. They collide and couple together. The amount of kinetic energy turned into heat (mostly warming the couplers) in the collision is...

a. 2500 J.
b. 10,000 J.
c. 1250 J.
d. 7500 J.
e. 11,250 J.

 

[phyzguy]

Try conserving momentum to figure out what the motion is after the collision. Then try to calculate the kinetic energy after collision and compare to the kinetic energy before the collision.