1. A mass of 5kg moves horizontally across a smooth surface at 1ms-1. Another mass of 5kg is lowered gently onto the first mass. a) What is the change in velocity of the mass? b) What impulse has been applied to the mass? c) Is this an elastic 'collision'? 2. force x time = change in momentum p=mv 1/2mv2 3. a) I think there is no change in velocity, as the mass is added downwards gently and it is a smooth surface so no friction force need be considered. b) p=mv Doubling the mass, doubles the momentum. Doubling the momentum, doubles the impulse. c) Is this a collision, as the mass is lowered gently? If it is, then I think it must be an inelastic collision as Total Kinetic Energy before Collision =! (does not =) Total Kinetic Energy after Collision - 1/2mv2, as the mass has been doubled. Any good?