Hi, 1. The problem statement, all variables and given/known data I'd like to find the minimum mass M which will render possible a second collision between M and the two masses m and the spring in the attachment, and also the time between the two collisions. It is stated that the initial velocity of M is v0 as indicated and that the surface is frictionless. It is also stated that this is an elastic collision, whose time is very short. M doesn't get attached to the masses. 2. Relevant equations 3. The attempt at a solution I am not quite sure how to formulate a condition on M, which will enable a second collision. I am not even sure how a second collision would be possible unless there were let's say a wall on the other side hitting M back towards the masses and the spring. The kinetic energy of M is turned into an elastic energy of the spring and kinetic energy of both M and the reduced mass = m/2, right (due to conservation of energy)? Furthermore, there is conservation of linear momentum, isn't there? Must I work in the CM reference frame?