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Homework Help: Tension and Pulleys theory question

  1. Oct 4, 2015 #1
    1. The problem statement, all variables and given/known data

    1. 17. Give the correct tension, T, in the marked rope for each situation. The rope is held so the system is stationary. All ropes and pulleys are massless and frictionless.

      A) T = W B) T = W/2 C) T = W/3 D) T = 2W E) T = 3W/2
      F) T = 2W/3 G) T = W/7 H) T = W/5 I) T = 2W/7 J) T = 2W/5

      1) pulley01.1.gif 2) pulley01.14.gif
      3) pulley01.9.gif 4) pulley01.10.gif

    2. Relevant equations

    No equations, mostly just theory.

    3. The attempt at a solution

    I've been trying to look at the various forces on each pulley compared to the next to solve this problem, as well as draw free body diagrams, but I don't feel like I'm getting anywhere, and I'm unsure of how the force will transfer from one pulley to the next, as all of the pulleys are connected. Any guidance on how to better understand the way the force changes from one pulley to the next? Thanks in advance.
  2. jcsd
  3. Oct 4, 2015 #2


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    Homework Helper

    Since each pulley is massless and frictionless, then the tension in one line must be equal to the tension in the other line.

    Take 1) above. If you make a FBD of the weight W and the sheave to which it is attached, what must the tension be in the two lines supporting that sheave?
  4. Oct 5, 2015 #3
    If you are not familiar with the simple machine called "block and tackle", I would suggest a Google search on the term.
    The rule of thumb for this device is that the mechanical advantage is equal to the number of ropes (or chains) supporting
    the bottom pulley. This should help you getting starting thinking on this type of problem.
    The following Web Site shows some interesting examples: http://www.lhup.edu/~dsimanek/TTT-fool/fool.htm

    fool-mec2.jpg fool01c.gif
    Fig. 2. Classic fool's tackle.
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