Energy problem with inelastic collision

[mickellowery]

Homework Statement

A railroad car with a mass 2.50x10^4 kg moves at 4.00 m/s. It collides and connects to 3 more cars each with the same weight as the first, they're moving 2.00 m/s. After they connect the final velocity was 2.50 m/s. How much energy is transformed into internal energy in the collision?

Homework Equations

I was trying to use [tex]\Delta[/tex]E=[tex]\Delta[/tex]K+[tex]\Delta[/tex]U=-f_kd. I wasn't sure if this was the right approach, and I think maybe I'm setting everything up wrong because I'm not coming up with the right answer.

The Attempt at a Solution

 

[rock.freak667]

Homework Equations

I was trying to use [tex]\Delta[/tex]E=[tex]\Delta[/tex]K+[tex]\Delta[/tex]U=-f_kd. I wasn't sure if this was the right approach, and I think maybe I'm setting everything up wrong because I'm not coming up with the right answer.

The change in kinetic energy should give the work done which should be the same as the change in internal energy.