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Simple Harmonic Motion find Mass

  1. Apr 15, 2012 #1


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    1. The problem statement, all variables and given/known data
    A block is attached to a horizontal spring with k = 75 N/m undergoes SHM with an amplitude of 0.15m. If the speed of the block is 1.7 m/s when the displacement is 0.12m from equilibrium, what is the mass of the block?

    2. Relevant equations
    E = E'
    E[itex]_{k}[/itex] = [itex]\frac{1}{2}[/itex]mv[itex]^{2}[/itex]
    E[itex]_{s}[/itex] = [itex]\frac{1}{2}[/itex]kx[itex]^{2}[/itex]

    3. The attempt at a solution
    E = E'
    E[itex]_{k1}[/itex] + E[itex]_{s1}[/itex] = E[itex]_{s2}[/itex]
    [itex]\frac{1}{2}[/itex]mv[itex]^{2}[/itex] + [itex]\frac{1}{2}[/itex]kx[itex]^{2}[/itex] = [itex]\frac{1}{2}[/itex]kx[itex]^{2}[/itex]
    Where subscript 1 indicates that x = 0.12 m and subscript 2 indicates that x = 0.15 m (max energy)
    Substituting in ...
    m = 0.21 kg

    but the answer is 0.27 kg?
  2. jcsd
  3. Apr 15, 2012 #2


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    Gold Member

    With that data, the answer is m = 0.21 kg.
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