What Is the Mass of the Caboose After the Collision?

[MooCow]

Homework Statement

A 29000-kg railroad freight car collides with a stationary caboose car with brakes released. They couple together, and 10 percent of the initial kinetic energy is dissipated as heat, sound, vibrations, and so on. What is the mass of the caboose?

This is the equation that I've gotten. However, there are to many variables so I don't know where to go from here. I plugged in the same velocities but the mass turned out negative.

m1v1=(m1+m2)vf

I posted this question exactly as it was written. I wish I had more information for those who asked for it, but that is all I was given- thus my confusion!

Any help would be great. Thanks.

 

[learningphysics]

You've got the first equation... you need one more... write the energy equation using the kinetic energies before and after the collision...

Then you'll have 2 equations with 2 unknowns (vf and the mass of the caboose).