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Homework Help: Calculating Mu from a counterbalanced inclined plane

  1. Feb 26, 2006 #1
    The physics homework for the weekend dealt with a lab we did the previous day. Currently I'm stuck, and wondering if anyone can help direct me in the right direction.

    The lab consisted of an inclined plane with an attached pulley. We hooked up a rolling cylinder to a string, that went over the pulley, and on the other side we added weights to balance out the system. The next step was to slowly add weight until we could get the cylinder rolling up the plane at a nearly constant (or as close as we could get it) speed, and writing down how much mass we used to get the cylinder to move at that rate.

    We students did this for a variety of angles for the plane, and took down just the mass of the counterbalance each time.

    Now, our assignment for the weekend was to calculate mu from the data we took from the lab. We assumed that the pulley and string were both massless and frictionless. My train of thought (up to where I became stuck)

    For the cylinder:

    Sigma(F-subx) = m*a-subx
    Sigma(F-subx) = 0 (since a-subx is 0, assuming nearly constant velocity)

    T - m*g*sin(theta) - Friction = 0
    T - m*g*sin(theta) - mu*m*g*cos(theta) = 0 (taken from vertical components of FBD)

    So I get mu by itself and have:

    mu = (T - m*g*sin(theta)) / (m*g*cos(theta))

    Now, I have T, m, and mu that I do not have values for. I can find the numerical value for T -- (the weight of the counterbalance -> m*g). Even so, I'm still stuck with m without a value, and I don't really know how I can get m to cancel out, or to find a suitable substitute to solve the equation.

    Any help would be appreciated. Thank you for your time.

  2. jcsd
  3. Feb 27, 2006 #2
    You forgot one crucial step: Do a Newton's Law problem on the counterbalance mass. That will give you a value for T. As far as m is concerned, if you are doing the problem the way you state it then you need to have measured it. The only other way involves graphing and you didn't mention that.

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