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Homework Help: Rotating mass connected to elastic spring (help needed)

  1. May 8, 2012 #1
    1. The problem statement, all variables and given/known data

    A mass m connected using elastic spring to the roof.
    the mass is rotating horizontaly.


    mass m= 0.6kg
    rate or spring constant (k) = 40 N*m-1
    frequency= 1/2 sec-1
    spring's equilibrium length is 0.8m

    The question is:
    what is the length (L) of the spring while the mass rotating?
    (earths gravity of course)

    2. Relevant equations

    F(spring)= -kΔx
    Fr= mω2r

    3. The attempt at a solution

    What I did is:
    On y: Tsinθ=mω2r
    now r=L*sinθ
    And T = kΔx = k(L-0.8)
    ω = 2*pi*f = pi
    → k(L-0.8)*sinθ=mω2*L*sinθ

    →→ L=0.939

    Now the problem is, that I believe that my colculation proves that
    gravity have no effect on the length of the spring, and I can hardly
    believe that, In fact Im almost sure that Im wrong.

    Hope you can help me find my mistake!
  2. jcsd
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