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Homework Help: Conservation of Energy or Momentum?

  1. Apr 28, 2007 #1
    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.
  2. jcsd
  3. Apr 28, 2007 #2
    Note here W(1) A(1) stands for initial amplitude and initial angular velocity.
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