Speed and Mechanical Energy Lost in Railroad Car Collision

[TrippingBilly]

A railroad car of mass 2.49 104 kg, is moving with a speed of 3.89 m/s. It collides and couples with three other coupled railroad cars, each of the same mass as the single car and moving in the same direction with an initial speed of 1.95 m/s.
(a) What is the speed of the four cars after the collision?
(b) How much mechanical energy is lost in the collision?

Sorry, but I have no idea about this problem :(

 

[Doc Al]

Hint: What is conserved in any collision?

 

[TrippingBilly]

Momentum, so...

2.49e4 * 3.89 + 3*2.49e4 *1.95 = 4*4.29e4 * V

?

 

[Doc Al]

Momentum, so...

2.49e4 * 3.89 + 3*2.49e4 *1.95 = 4*4.29e4 * V

Right. Looks good, except for a typo. Once you solve for the final speed, you can figure out the initial and final kinetic energies and compare them.