1. A railroad car loaded with rocks coasts on a level track without friction. A worker on board starts throwing the rocks horizontally backward from the car. Then what happens? (a) car slows down (b) car speeds up (c) first the car speeds up and then slows down (d) car's speed remains constant 2. Which answer would you choose if the rocks were to fall out through a hole in the floor of the car one at a time? So the answer to number 1 is (b) speeds up and the answer to number two is (d) car's speed remains constant. I understand number one but I can't understand why number two would be (d) using the following equation for momentum: Pi=Pf Here's what I was thinking please tell me where I'm going wrong: (c=car and r=rock) McVc + MrVr = McV'c + MrV'r Initially Vc=Vr=Vi and since rocks are being dropped directly downward, final momentum of the rocks (being dropped one at a time) should be zero because their falling down, out of the railroad car. So: Vi( Mc+Mr) = McV'c V'c = [Vi(Mr+Mc)] / Mc But wouldn't that increase the velocity? I know logically thinking it wouldn't but I can't seem to get this equation to show that. Someone please help explain this to me!!