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Homework Help: Two Blocks, Massless Rope. Pulley

  1. Oct 17, 2007 #1
    1. The problem statement, all variables and given/known data

    The two blocks, m1 = 3.3 kg and m2 = 4.2, in the figure below are connected by a massless rope that passes over a pulley. The pulley is 12 cm in diameter and has a mass of 2.0 kg. As the pulley turns, friction at the axle exerts a torque of magnitude 0.35 Nm. If the blocks are released from rest, how long does it take the 4.2 kg block to reach the floor from a height of h = 1.0 m?

    2. Relevant equations

    torque= r * F
    sum of torques = moment of inertia * angular acceleartion
    angular acceleration * radius = acceleration
    x=v0t + 1/2 a t^2

    3. The attempt at a solution

    I tried summing torques, saying that m2 is on the left and has torque m2*g*radius. Following this, m1 acts opposite, so subtract m1*g*radius. Friction acts opposite of motion, so acts the direction of m1. So (m2*g*r) - (m1*g*r) - torque(friction) = alpha * 1/2m*r^2 (m is mass of pulley, r is .06m, m1 = 3.3, m2=4.2)

    from here, i used alpha*r=a to find linear acceleration

    then i used x=v0t + 1/2at^2 to find time, v0 is 0 cuz it starts at rest, a is what i found before

    Any suggestions?
  2. jcsd
  3. Oct 17, 2007 #2


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    Homework Helper

    You need to examine each part separately... the mass on one end of the pulley... the mass on the other end... and the pulley itself...

    Write the f = ma equations for each of the 2 masses...

    Write the torque = I*alpha equation for the pulley...

    The two tensions cannot be assumed to be the same... so assume two variables T1 and T2...

    The acceleration of one mass upward will equal the acceleration of the other mass downward.

    alpha = a/0.06

    you'll have 3 equations with 3 unknowns. a, T1, T2...
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