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Homework Help: Work done by motor against gravity

  1. Nov 17, 2015 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations
    Conservation of Energy: ∑T1+∑U1-2=T∑2

    3. The attempt at a solution

    (600+2s2)ds - 100*9.8*15 = (1/2)mv2.

    However, the term of work done by the force is less than the term of work done by gravity! The book has a solution, which is exactly this except the term of work done by the motor is multiplied by 2. Could it have something to do with the pulley? Otherwise, I don't get it.

  2. jcsd
  3. Nov 18, 2015 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Well, the motor is shown connected to the weight by means of the pulley, so you can't assume that the pulley is not there.

    You should draw a free body of the sheave and see how its presence affects the force provided by the motor and on the weight being lifted.
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