Conservation of Energy or Momentum?? A mass M is attatched to a horizontal spring of force constant k fixed on one side to a rigid suppport.(Please imagine the diagram). The mass oscillates on a frictionless surface with time Period T and amplitude A. When The mass is in equilibrium another mass m is gently placed on it. What will be the new amplitude of oscillation. My approach, Conservation of energy 1/2 Mw(1)^2A(1)^2= 1/2 (M+m)W(2)^2A(2)^2 where w is omega. And then on rearranging the terms i got [A(1)/A(2)]^2 and w(1)/w(2) on the left hand side i substituted it with (m+M)/(M) since w is inversely proportional to m. This did not lead me to the right answer. However if i conserved momentum then i do get the right answer. Question When am i supposd to know when to conserve momentum and when to conserve energy. Anyone to help since my AIEEE examination is tomorrow.