posted by Angela .
A railroad car with a mass of 1.86 104 kg moving at 2.80 m/s joins with two railroad cars already joined together, each with the same mass as the single car and initially moving in the same direction at 1.40 m/s.
(a) What is the speed of the three joined cars after the collision?
(b) What is the decrease in kinetic energy during the collision?
The answer to (a) is 1.27, but I can't get (b)!!