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Calculating the mass of the wheel on a pendulum on a watch

  1. May 4, 2013 #1
    1. The problem statement, all variables and given/known data
    The balance wheel of a watch is a thin ring of radius 0.95 cm and oscillates with a frequency of 3.10 Hz. If a torque of 1.1x10-5 Nm causes the wheel to rotate 45°, calculate the mass of the balance wheel.


    2. Relevant equations
    I=mr2
    T=(2π)sqrt(I/mgh)
    τ=-Kθ



    3. The attempt at a solution
    I got an answer but it's wrong...

    Here's the work I did:
    1.1e-5=-K(pi/2)
    K=-7.003e-6
    19.5 rad/s = sqrt (7.003e-6/(M(0.0095m)2))
    380.25 rad/s2 = 7.003e-6 Nm / 9.025e-5m M
    M*0.0343 m/s2 = 7.003e-6 Nm
    m = 2.04e-4 kg

    The answer should be 0.41g
    (I realized just now that I took the square root of a negative number earlier. I tried to redo the problem but I'm still stuck...) Thanks!
     
  2. jcsd
  3. May 4, 2013 #2

    SammyS

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    Is that the complete problem, word for word.

    It seems that some information is missing. I could guess at what's missing, but it would be just a guess.
     
  4. May 4, 2013 #3
    Yep, that's the question word for word.
     
  5. May 4, 2013 #4

    TSny

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    What is 45o in radians?
     
  6. May 4, 2013 #5
    pi/4
     
  7. May 4, 2013 #6
    Oh my goodness... I just realized why you were asking me that. Wow, I am dumb. Man, those stupid mistakes get me every time. Thanks for the help.
     
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