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Homework Help: Simple Harmonic Motion for a block-spring system

  1. Jun 23, 2012 #1
    1. The problem statement, all variables and given/known data

    If the phase angle for a block-spring system in SHM is π/5 rad and the block's position is given x = xm cos(ωt + phi), what is the ratio of the kinetic energy to the potential energy at time t = 0?

    2. Relevant equations

    PE = (1/2)*k*[(max amplitude)^2]*cos^2(ωt + phi)
    KE = (1/2)*k*[(max amplitude)^2]*sin^2(ωt + phi)

    Solve for KE/PE in that form.

    3. The attempt at a solution

    I took KE and divided by PE, giving sin^2(phi)/cos^2(phi). Plugging in pi/5 for phi gives us 1.20253316E-4/.99987974669. However, the system is saying this is wrong . I do believe that I solved for KE/PE correctly. Any help would be much appreciated. Thanks very much!
  2. jcsd
  3. Jun 24, 2012 #2


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    Homework Helper

    Have you set your calculator to RAD? pi/5 is in radians.

  4. Jun 24, 2012 #3
    I knew I was doing it right! my calculator was in degrees....i hate it when that happens...thanks for helping me figure that out!
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