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Homework Help: Work on 1 pulley system and two masses

  1. Feb 3, 2016 #1
    1. The problem statement, all variables and given/known data
    this is from an example problem in the hibbler dynamics book 13th edition in the section of work so i dont really have a question on how to do it. the question is why is the problem solved the way it is solved.

    the system has one pulley.one rope.and two masses at each end of the rope the work done by gravity is positive for both masses....why?. since one mass is going up and the other mass down. shouldn't work by gravity be negative for the mass going up?
    i assign positive going down.

    2. Relevant equations
    work done on an object = the change in the objects KE

    3. The attempt at a solution
    i solved with kinematic equations and found the answers match. so I know its correct to assume that both masses go down with energy equations, but if I try to solve with one mass going up (and so work by gravity is negative on that one)and the other down (so work by gravity is positive on this one) using energy equations I get a different/wrong answer. so why must both masses be assumed to move down? is it because of the rope? How? also, tension does zero work. Why? is it because its a reaction force? can reaction forces do work?
  2. jcsd
  3. Feb 3, 2016 #2
    Can you please provide a diagram and show us your work? Thanks.
  4. Feb 3, 2016 #3
    this more of a concept question not a how to do it question since this is an example problem and the solution with step by step solution is already provided. all im asking is 1.)why must it be assumed that both masses move down and how can it be done with 1 mass going up and the other down using cons. of energy. 2.) why doesnt the tension in the cord do work.3) do reaction forces do work? if so whats an example of one?
  5. Feb 3, 2016 #4
    I agree with you. For one of the masses, the potential energy increases, and for the other mass, the potential energy decreases. Any possibility of your typing out the solution they gave so that I can see exactly what they did? It's very hard to address your question without seeing more details.

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