Dropping coal on a platfrom (energy cons)

[mujadeo]

Homework Statement

A railroad hopper car weighs 20000 kg when empty and contains 10000 kg of coal. As it coasts along the track at 9 m/s the hopper opens and steadily releases all the coal onto a platform below the rails over a period of 5 s.

Homework Equations

d) If the coal fell 5 m, what was the total energy converted to heat in the process of the coal coming to rest?

The Attempt at a Solution

Ive already got the first 3 parts of this question. they are
How fast does the car travel after all the coal is dumped?
=9m/s

What was the momentum absorbed by the platform?
=9000kg/m/s

If the coal comes to rest 1 second after the dumping is finished, what was the average horizontal force on the platform?
=9000N

My question b4 attemptin solution is --how can energy be conserved if coal is at rest? (no potential, and no kinetic).
If it cant, then that would mean all the energy must have gone somewhere.. --heat.

So 9000N*5m = 45000J.
This is wrong.
??

 

[mgb_phys]

You are correct about energy being conserved, what two types of energy does the coal have when it is moving in the train?
When it comes to rest below the train it loses both of these - to heat as you said.

 

[mujadeo]

AHA potential AND kinetic because car moving 9ms
so Ek + Ep = 895000J

thanks!