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Homework Help: Double Pulley Mechanics Problem

  1. Oct 13, 2014 #1
    1. The problem statement, all variables and given/known data
    it is a double pulley problem, mass of pulley 1 is 4.1kg, mass of pulley 2 is 6.9kg and mass of crate is 57.2kg.
    both pulleys are frictionless. the angle at which the rope is pulled is 45deg. What is the minimum tension with which the operator must pull on the cable (negligible mass) in order to slowly raise the crate?
    2. Relevant equations
    i was wondering if this was a torque problem of simply a one dimensional problem involving pulleys? also do we need to use newtons second law?

    3. The attempt at a solution
    I tried ignoring the masses of the pulleys since we have not yet learn torque but the answer was wrong. however the radius of the pulleys are not given so i am not sure i can use torque for this problem?

    Here is the image : https://s.yimg.com/hd/answers/i/ff7b49a654fb4768a4d9902f8dc5ac60_A.png?a=answers&mr=0&x=1413224583&s=0388b130543c86fed3259f977c0872ce [Broken]
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Oct 13, 2014 #2


    Staff: Mentor

    I believe you can ignore pulley 1, as it is not being lifted, but you have to include pulley 2. Pulling on the cable lifts both the crate and pulley 2. The vertical component of the tension will be equal in magnitude to the downward force that comes from the crate and pulley 2. You need to figure out what the tension T is.

    BTW, this is not an advanced physics problem, so I moved it to the Intro Physics section.
    Last edited by a moderator: May 7, 2017
  4. Oct 13, 2014 #3


    User Avatar
    Science Advisor
    Homework Helper
    2017 Award

    No need for torques, just forces is enough.
    Make a drawing with the forces working on the masses. Which ones have to be dealt with by the rope ?
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