Dropping coal on a platfrom (energy cons)

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.
??