1. The problem statement, all variables and given/known data A railroad hopper car has a mass of 20000 kg when empty and contains 30000 kg of coal. As it coasts along the track at 7 m/s the hopper opens and steadily releases all the coal onto a platform below the rails over a period of 5 s. a) How fast does the car travel after all the coal is dumped? b) What is the horizontal impulse delivered to coal by the platform? c) If the coal comes to rest 1 second after the dumping is finished, what was the average horizontal force on the platform? d) If the coal fell 4 m, what is the total energy converted into heat in the process of the coal coming to rest? e) Let's say that instead of the coal falling out of the bottom of the coal car, some very strong physics student threw it backwards from the car, so that the velocity of each piece of coal was zero just after it left her shovel. How fast does the car travel after all the coal is thrown instead of dumped? 2. Relevant equations p = mv pi =pf 3. The attempt at a solution The answer for part a was 7 m/s, so apparently the platform was attached to the hopper, but I have no idea where to begin with solving any of the others. Please help.