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Homework Help: Potter wheel

  1. Nov 19, 2004 #1
    So I'm doing an online homework problem:

    A potter's wheel - a thick stone disk of radius 0.405 m and mass 112 kg - is freely rotating at 48.0 rev/min. The potter can stop the wheel in 5.67 s by pressing a wet rag against the rim and exerting a radially inward force of 69.1 N. Find the effective coefficient of kinetic friction between the wheel and rag.

    Round your answer to three significant figures.

    [tex] f = \frac {I \omega_i}{r(\Delta t)} = \frac {(112 kg) (0.405 m^2)(48.0 rev/min)} {(0.405 m)(5.67 s)} \left(\frac{2\pi rad}{1 rev}\right) \left( \frac {1 min}{60 s}\right) = 40.2 N[/tex]

    [tex] \mu_k = \frac {f}{n} = \frac {40.2}{69.1}=0.582 [/tex]

    Trouble is, I keep getting "WRONG".

    Am I doing it wrong? I also tried with different # of significant digits and nothin. Any help would be great.
  2. jcsd
  3. Nov 19, 2004 #2
    Looks like you're using mr^2 for the moment of Inertia...which would work if it were a ring..

    ..however, the moment of Inertia of a thick solid disk is:

    I = 0.5mr^2..

    .....hope that helps!
  4. Nov 19, 2004 #3
    Right right right... ok, thanks. I just got it after I posted this. heh heh
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