My first post here! I signed up a few minutes to correct someone... then deleted my post when I realised I was on about something entirely different. Ahem. Anyway, have a look at this diagram: http://img2.freeimagehosting.net/uploads/c8911ecf7e.jpg Particle P is suspended on an elastic string fixed at O, natural length = 'l', equilibium point is ( l + d ) below O, and 'x' is the distance the particle has been pulled down to. Correct me if this is wrong. If you release P its motion will be simple harmonic, at least until it reaches the equilibrium point. But what happens to its motion after that? Once " OP < l ", its motion is like a normal particle under gravity but how does it move when " l < OP < l + d "? Thanks in advance!