Lets say you have a ball on a ramp (hight h). It rolls down to the ground and then starts moving at aconstant speed - no friction (ok don't kill me, it's not really rolling either, just sliding). Now to the point of view of someone moving at the final speed of the ball and in the same direction, lets call it v, the ball starts with hoight h and with speed -v, and at the end it has hight 0 and speed 0. Were did all the energy go? I think that since the ramp can't have infinate mass it must go a little backwards as the ball goes forwards (momentum conservation), in the first case the ramp starts with 0 speed and then gets a little from the ball, and in the second case the ramp starts with -v and then gets more from the ball so the difference is much bigger (for example, 11^2 - 10^2 > 2^2 - 1^2). This difference in energy is equal to the energy lost by the ball. (I calculated it and it worked) Is this the right answer? What happens if the ramp has infinate mass(theoreticly)? Thanks.