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Homework Help: Simple Harmonic Motion Energy

  1. Apr 2, 2009 #1
    1. The problem statement, all variables and given/known data
    A mass "m" is attached to the free end of a light vertical spring (unstretched length L) of spring contant "k" and suspended from a ceiling. The spring streches by Delta L under the load and comes to equilibrium at a height "h" above the ground level (y=0). The mass is pushed up vertically by "A" from its equilibrium position and released from rest. The mass-spring executes vertical oscillations. Assuming that gravitational potential energy of the mass "m" is zero at the ground level, show that the total energy of the spring-mass system is (1/2)k{Delta L^2 + A^2} + mgh.

    2. Relevant equations
    x(t)= Asin(Omega t + phi)
    v(t)= wAcos(Omega t +phi)
    Total Energy = 1/2 k A^2
    Delta L k = mg

    Hint: Draw four sketches of the vertical free spring, spring-mass in equilibrium, and the two extreme positions of oscillation of the mass. Below each of the above sketches (except free spring) write equations for Kinetic Energy, Elastic Potential Energy, Gravitational Potential Energy and Total Energy using symbols k, Delta L, m, A, g, h and v_m_

    3. The attempt at a solution
    I am struggling trying to find where Delta L fits into this equation. I understand the change in energies at the top and bottom are both full PE and GravPE and while it passes through equilibrium the KE is max because velocity is max. I just don't know how to even go about this problem I guess. I've labeled my drawing with the differences in h where at equilibrium y=h and the top y=h+A and at the bottom y=h-A. I'm lost. HELP please. Thank you in advance.
  2. jcsd
  3. Apr 2, 2009 #2
    I have attempted a bit more to this and I believe at equilibrium the total energy of the system is split up into three parts: Kinetic Energy is 1/2 m v_max_^2 + Elastic Potential Energy which is 1/2 k Delta L^2 + mgh

    This is the answer if I sub in Omega and such and cancel masses I end up with the answer. But this is only the answer for equilibrium and I can't prove it is true at the extrema.

    Any ideas?
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