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Homework Help: Atwood Problem

  1. Oct 19, 2014 #1
    1. The problem states:

    Problem 9-72a:
    The system shown in the figure below consists of a m1 = 4.24-kg block resting on a frictionless horizontal ledge. This block is attached to a string that passes over a pulley, and the other end of the string is attached to a hanging m2 = 2.12-kg block.
    http://loncapa.mines.edu/res/whfreeman/tipler/Physics_for_Scientists_and_Engineers_6e/Chap09/graphics/tipler9-68.gif [Broken]

    The pulley is a uniform disk of radius 8.19 cm and mass 0.565 kg. Calculate the speed of the m2 = 2.12-kg block after it is released from rest and falls a distance of 2.23 m.

    Problem 9-72b:
    What is the angular speed of the pulley at this instant?

    2. Relevant equations

    3. The attempt at a solution

    I set my system to be both the masses and the pulley, therefore the only external force would be the force of gravity. I think I'm supposed to set that equal to the translational and rotational energies of the system, translational for the masses and rotational for the pulley. But i don't know what they equations for the translational and rotational energies would be. Once i figure that out i can solve for the second part of the problem. Thanks in advance!
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Oct 19, 2014 #2


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    You cannot equal force with energy.

    The problem can be solved by considering the conversion of one type of energy into another. You quote one relevant equation for energy, and -for this problem- you need three... Look them up in your book...
  4. Oct 19, 2014 #3
    Use conservation of energy:
    [itex] K_i + U_i = K_f + U_f [/itex]
    Remember that there are two types of energies in this problem (translational and rotational)
  5. Oct 19, 2014 #4
    To go a step further, conservation of energy will give an equation like:
    [itex] \frac{1}{2}I\omega^{2} + \frac{1}{2}M{v_1}^2+ MgH_1 = \frac{1}{2}M{v_2}^{2} + \frac{1}{2}I\omega^{2} + MgH_2 [/itex]
    Last edited: Oct 19, 2014
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