I was looking through the revision section of my textbook in preperation for my test tomorrow, when I came across an interesting problem, which I can't manage to solve. The problem is: A body of mass 2kg collides elastically with another body that is stationary. The moving mass continues on in it's initial direction, at 1/4 of its initial velocity. What is the mass of the stationary object? I realise that the conservation of momentum, and the conservation of kinetic energy must be used to solve this problem. I can solve it for any specific case (ie. substitute in a value for the velocity of the first object) but not generally, although it should be the same answer. I always end up with 2 equations and 3 unknowns that I cannot cancel out. Thanks for any help.