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Pulley question / rotational kinematics

  1. Sep 10, 2011 #1
    1. The problem statement, all variables and given/known data

    The pulley in the figure has radius (R) and a moment of inertia (I). The rope does not slip over the pulley, and the pulley spins on a frictionless axle. The coefficient of kinetic friction between block A and the tabletop is [itex]\mu[/itex]k. The system is released from rest, and block B descends. Block A has mass (ma) and block B has mass (mb).

    Use energy methods to calculate the speed of block B as a function of the distance (d) that it has descended.
    Express your answer in terms of the variables ma, mb, R, I, [itex]\mu[/itex]k, d and appropriate constants

    2. Relevant equations
    Conservation of Energy:

    PEi + KEi = PEf + KEf + frictional work

    3. The attempt at a solution

    (magd) + (mbgd) = maVa2/2 + mbVb2/2 + I[itex]\omega[/itex]2/2 + [itex]\mu[/itex]kmg

    0 + (mbgd) = 0 + mbVb2/2 + I[itex]\omega[/itex]2/2 + [itex]\mu[/itex]kmg

    Is this right?

    Attached Files:

  2. jcsd
  3. Sep 10, 2011 #2


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    Two questions
    1. Why is this zero?
    2. Which mass is this?
  4. Sep 10, 2011 #3
    1. I don't know. I thought maybe that the velocity of a is non existent, but wouldn't the velocity of a equal the velocity of b?

    2. I would say since the frictional work done is between the surface and object a, that mass should be the mass of block a.

    Thank you for your help
  5. Sep 10, 2011 #4


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    If the velocity of a were zero and the velocity of b were not, the distance between the two blocks would increase in which case the rope connecting them would stretch. This is not what happens. If the rope is not to stretch (or shrink), the two blocks must always have the same instantaneous velocity and acceleration.

    Correct. Now put it together.
  6. Sep 11, 2011 #5
    Thank you for the help. I applied what you said and got the following answer:


    What have I done wrong?


    Attached Files:

  7. Sep 11, 2011 #6


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    Look at your numerator under the radical. You are adding two terms that are dimensionally inconsistent. What are the dimensions of each term?
  8. Sep 11, 2011 #7
    I think what you're saying is that I have:

    [(m^3) / (s^2)] - [(m^2) / (s^2)]

    So am I missing the d variable? So should it be:
    (m_B * g * d) - (u_k * m_A * g * d)
  9. Sep 11, 2011 #8


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    Yes, that's what the numerator under the radical ought to be.
  10. Sep 11, 2011 #9
    Thank you so much!! :smile:

    That's the right answer!
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