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Homework Help: Coefficient of dynamic friction

  1. Nov 2, 2007 #1
    1. The problem statement, all variables and given/known data

    A 4 kg collar C rides along a horizontally rotating arm AB. A spring k connects C to A. The arm AB rotates at a constant rate of 15 rpm. At the illustrated instant r = 1 m, r˙ = 1.1 m/s, and r¨ = 4 m/s2. The coefficient of dynamic friction, μk, is equal to 0.3. What is the magnitude of the total force exerted by the spring on the collar?

    http://img337.imageshack.us/img337/958/dynamicsqig9.th.gif [Broken]

    2. Relevant equations

    [tex]a=(\ddot {r} -r \dot {\theta}^2)i_R + (2 \dot {r} \dot {\theta} + r \ddot {\theta})i_ {\theta}[/tex]

    3. The attempt at a solution

    [tex]\\sumF_r=ma_r=m(\ddot {r} -r \dot {\theta}^2)=-F_s-\mu R[/tex]
    [tex]\\sumF_{\theta}=ma_{\theta}=m(2 \dot {r} \dot {\theta} + r \ddot {\theta})=R[/tex]

    Everything is known exepth for [tex]F_s[/tex] which is the answer (force on spring), R, and [tex]\dot {\theta}[/tex].

    [tex]\dot {\theta}=\frac {rpm*2 \pi}{60} = 1.57[/tex]

    I plug in numbers and get R=13.82 and Fs=-10.27

    Any help would be appreciated
    Last edited by a moderator: May 3, 2017
  2. jcsd
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