Here is the problem as it reads out of the book. In the figure, puck 1 of mass .2 kg is sent sliding across the table [frictionless] to undergo a one-dimentional elastic collision with stationary puck 2. Puck 2 then slides off the edge and lands a distance d from the base of the table. Puck 1 rebounds from the collision and slides off the opposite edge of the table, landing a distance of 2d. What is the mass of puck 2? The figure mentioned in the question is fairly well described in the problem, it is a leven, frictionless table with 2 pucks on it. Here is what I make of the question. since puck 2 slides a distance "d" from the table while falling and puck 1 slides "2d", I conclude that puck 1 has twice the horizontal velocity, although negative) as puck 2 when it left the table (and also just after the collision since the table is frictionless). Since puck 1 is sliding one direction at the start of the problem, stops and then reverses direction and has a larger speed, its mass must be smaller than that of puck 2. Since this is an elastic collision, conservation of kinetic enery and conservation of momentum apply. I will make a guess that puck two has twice the mass (.4 kg) of puck 1, thus casing it to have half the kinetic energy aftter the collision. Although I dont know how to prove this, mathmatically, how do I show it?