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Homework Help: Forces over a disk

  1. Nov 10, 2012 #1
    1. The problem statement, all variables and given/known data


    The pulley(disk) has a radius R and a mass m 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 surface is μk. the system is released from rest and block B descends. black A has mass 2m and block B has mass m

    given [R m h μk]

    find the speed of block B after falling distance h
    the time to drop distance h

    2. Relevant equations
    τ = I α

    Idisk = mR^2

    α = a/R

    3. The attempt at a solution

    I can do the basic non rotational forces no problem. I dont have any experience messing with rotational force but have seen the torque = I alpha formula.

    τ = mR^2 (a / R)
    = mRa

    i dont know how to relate this torque(assuming it's even right :p) back into x y forces in the form of tensions to tie the entire problem together. I am fairly comfortable with using energy but I need to be comfortable with using forces as well and this question is asking for time anyways, so it will likely make things easier in the long run... Thanks for any help.
  2. jcsd
  3. Nov 10, 2012 #2
    To find the speed, you don't need to think about the forces, just the energy. What is the energy of a disk spinning about it's axis?
  4. Nov 10, 2012 #3
    1/2 I omega^2

    I want to do it by forces for the sake of knowing how. need to know how to solve both ways for tests.
  5. Nov 10, 2012 #4


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    Moment of inertia of a disk is mr^2/2.
    For the force method, draw free body diagrams of each block and the pulley, and use newton's 2nd law on each. For the pulley, its the 2nd law of rotational motion.
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