1. The problem statement, all variables and given/known data A railroad hopper car has a mass of 30000 kg when empty and contains 50000 kg of coal. As it coasts along the track at 5 m/s the hopper opens and steadily releases all the coal onto a platform below the rails over a period of 4 s. What is the horizontal impulse delivered to coal by the platform? 2. Relevant equations Pi=Pf J=change in p/change in t 3. The attempt at a solution I tried 40,000 (which is total mass*velovity)=15,000(which is car*velocity)x which i thought was the change in momentum so I divided that by 4 sec. This was wrong. Any ideas?