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Homework Help: Vibration of rigid body

  1. Jan 4, 2015 #1
    1. The problem statement, all variables and given/known data

    Rigid body with one spring.

    2. Relevant equations
    Torque equations ( M=I * "alpha")

    3. The attempt at a solution
    So I think I can solve this problem but there is only one thing I do not understand.

    When I am writing my torque equation I would think that the weight of the bar and the cube would affect the torque
    M = I * a + Mb * g *0.4 cos (theta) + Mw * g * 0.4 cos (theta) - k*0.4sin(theta) *0.4 cos(theta)
    but apparently the equation should be
    M = I*a -k*0.4sin(theta)*0.4cos(theta)

    why does the mg of both the objects not affect the torque, whenever I try to include them the differential equation becomes unsolveable

    Attached Files:

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  2. jcsd
  3. Jan 4, 2015 #2


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    Homework Helper

    From the equilibrium position, your forces would be along the lines of kδ=mgcosθ (component of the weight)

    So that when the mass is displaced again by some distance x, the spring force becomes k(x+δ). And the weight cancels out.
  4. Jan 4, 2015 #3
    Alright thanks for the answer. Think I understand this now :)
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