1. Question A system consists of two blocks, each of mass M, connected by a spring of force constant k. The system is initially shoved against a wall so that the spring is compressed a distance D from its original uncompressed length. The floor is frictionless. The system is now released with no initial velocity. (See picture) [part c] Determine the period of oscillation for the system when the left-hand block is no longer in contact with the wall. 2. Relevant equations period = 2(pi)sqrt(m/k) 3. The attempt at a solution The answer given is this: period = 2(pi)sqrt(M/(2k)) The explanation given is "m = reduced mass = M/2". I don't understand the explanation given. I can't visualize what happens to the right mass M after the left mass M leaves the wall. This is different from all spring problems I have seen, where one end is attached to a wall, so of course I suspect a different answer. Could someone show me the math involved to prove the period is reduced like this?