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Homework Help: Hanging mass from a beam

  1. Dec 17, 2008 #1
    1. The problem statement, all variables and given/known data
    1.A bucket of mass m is connected by a rope to a frictionless pulley of mass M. Use Newton’s 2nd law for torques and Newton’s 2nd law to find an equation for the acceleration of the bucket if it is released from rest and allowed to fall.

    2. Relevant equations

    3. The attempt at a solution

    so i need we need to find Normal force in both direction. i know that

    sum of Force in X direction is that o= N- Tcos(theta)

    From the problem i am not sure how to find the Tension and where the normal force in the Y direction would come from.
  2. jcsd
  3. Dec 17, 2008 #2


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    Isn't what you have is a F = m*g being retarded by an I*a/R where I is the moment of inertia?

    Apparently the frictionless part refers to the rotation of the pulley and not the friction of the rope on the pulley because if the rope is free to slip, what torque would there be?
  4. Dec 18, 2008 #3


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    Gold Member

    Take the tension T to be the reactant force

    Hint...it is 90 degrees to the axis of rotation.
  5. Dec 18, 2008 #4


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    Forces acting on the bucket are T and mg. Since bucket is moving downwards, net force acting on it is
    ma = mg - T.......(1)
    The tangential force actin on pulley is T.
    Therefore The torque on pulley is T*R = I* alpha = I*Ra .......(2)
    Substituting the expression for I and solving eq. 1 and 2 , find the equation for the acceleration.
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