In my physics class (School's over now, so I can't ask my teacher.) we had this interesting problem for our final. (No, this isn't homework) We didn't do elasticity, soo... I dunno. An astronaut (It's in space to eliminate air resistance and gravity- it IS high school physics :tongue:) has a rather massive ball. Almost as massive as himself. He goes to a completely hard, elastic wall to play catch. When he throws the ball, though the ball is massive, he still could catch it if he were on the other side to catch. However, it hits the elastic wall and bounces back with enormous momentum. It knocks him back into space... So, did the momentum double? Why? The mass did not change, and no force made it accelerate to a faster velocity... What made it change? How does elasticity interfere? The wall wasn't moving, so it can't push off more momentum.