# Homework Help: Conservation of Energy or Momentum?

1. Apr 28, 2007

### i.mehrzad

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. Apr 28, 2007

### i.mehrzad

Note here W(1) A(1) stands for initial amplitude and initial angular velocity.

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