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Homework Help: Tension of a Pulley System

  1. Nov 4, 2016 #1
    1. The problem statement, all variables and given/known data
    A rod of length L = 20 m and mass m = 50 kg juts out perpendicular to a wall. The rod is designed to pivot vertically about the point where it intersects the wall. A massless pulley is affixed to the wall a height h = 10 m above the rod. A cable is attached to the rod a distance d = h from the wall-rod intersection. The cable slides over the pulley without friction and attaches at the other end to a disk of mass m = 20 kg and radius r = 50 cm on the wall next to the rod. The cable is pulled by turning this disk.
    In the first Part of the question, I determined the Tension of the cable to be 693 N.
    I Cant figure out the second part of the question:

    "If the disk spins at 60 rpm, how long will it take to pivot the rod from horizontal to vertical?"

    2. Relevant equations

    3. The attempt at a solution
    I attempted to Solve for time using w= w0 + at
    First I found inertia using the equation: 1/2 mR^2 > I= 2.5
    Then I found alpha using: T=a*I=F*r > a= 138.6
    I plug these number into the equation (60 rpm = 6.283 rad/s): 6.283= 0 +138.6*t > T=0.04

    This number for time does not seem possible. What am i doing wrong?
  2. jcsd
  3. Nov 4, 2016 #2
    Do you have a picture of the figure that you can post? It looks like maybe you tried to, but it didn't come through.

    Also, could you please post the first question as it is worded.

    For the second question, it says the disk spins at 60 rpm. That sounds like a constant velocity? If so, then there should be no angular acceleration. But I don't feel like I have a complete understanding of the problem.
  4. Nov 4, 2016 #3


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    How fast does the cable move, in cm/s?
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