Mechanical Energy loss due to NC Forces: two railroad cars collide.

[Senjai]

[SOLVED-Thanks AL]Mechanical Energy loss due to NC Forces: two railroad cars collide.

SOLVED! - Thanks Al![/color]

Homework Statement

Two railroad cars each of mass 4800 kg and traveling @ 80 km/h collide head on and come to rest. How much thermal energy is produced in this collision?

Homework Equations

$$E_k + E_p = E^{'}_k + E^{'}_p + Q$$

The Attempt at a Solution

This problem does not use momentum. Energy considerations only.

I treat one car as a single entity.

Energy before = energy after.

There was only an initial kinetic energy at the beginning and no kinetic energy afterwards. so i assume all the energy was transformed to Q.

$$\frac{1}{2}mv^2 = Q$$

after finding 80 km/h = 22.2222... m/s

i solved and got 1.2 x 10^6 J, the answer is double that, 2.4 x 10^6 J, where did i go wrong?

 

[Doc Al]

There are two moving trains.

 

[Senjai]

OH! I only calculated the amount of thermal energy one train would release, and not the other. So the answer would be Q of both trains (KE of train one + the KE of train two) as it's released simultaneously ?? Ugh... i thought i'd isolate only one train because I wasn't dealing with momentum.. got my head stuck in dynamics :) Thanks a lot Doc!