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.
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?
What was the momentum absorbed by the platform?
If the coal comes to rest 1 second after the dumping is finished, what was the average horizontal force on the platform?
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.