# Coal car

huskydc
A railroad hopper car has mass 50000 kg when empty and contains 50000 kg of coal. As it coasts along the track at 10 m/s the hopper opens and steadily releases all the coal onto a platform below the rails over a period of 4 s.

How fast does the car travel after all the coal is dumped?

i applied the mv(initial) = mv (final) equation...

i got something like

(100000 kg) (10m/s) = (50000 kg) V final

apparently it's incorrect, any hints?

zwtipp05
How is that incorrect? That should be exactly what you get. Now just solve for V.

Gold Member
Technically... shouldn't it travel the same speed? There wasnt a momentum transfer and there's no readily apparent force acting on the car.

zwtipp05
Pengwuino said:
Technically... shouldn't it travel the same speed? There wasnt a momentum transfer and there's no readily apparent force acting on the car.
It should pick up speed. The momentum changed. Think of it as a reverse inelastic collision.

Gold Member
But there is no collision, no momentum has changed because there's no force.

The only force I coudl see is if there was one generated by the coal which means a force of 125000 kg*m/s^2 was generated for 4 seconds... which still arrives at 20m/s for some reason... Well I am lost.

huskydc
momentum is conserved, i kept getting 20 m/s

does the time, 4 sec matter in answering this question?

Gold Member
huskydc said:
does the time, 4 sec matter in answering this question?

It has to... there's some sort of force acting here and I think this is where it occurs (at the 4 seconds). Did you put the whole question in the post?

zwtipp05
Pengwuino said:
But there is no collision, no momentum has changed because there's no force.

The only force I coudl see is if there was one generated by the coal which means a force of 125000 kg*m/s^2 was generated for 4 seconds... which still arrives at 20m/s for some reason... Well I am lost.
There is no collision in an explosion (another reverse inelastic collsion), yet momentum changes.

Gold Member
An explosion?

When are the geniuses going to come to this thread and show us how this is actually done?

huskydc
may i ask what explosion? or collision? the only thing happening is that the cart is dropping off coal..

Homework Helper
Pengwuino said:
An explosion?

When are the geniuses going to come to this thread and show us how this is actually done?

...
Pengwuino said:
Technically... shouldn't it travel the same speed? There wasnt a momentum transfer and there's no readily apparent force acting on the car.