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Pulley system with one mass and apparently independent forces?

  1. Aug 2, 2013 #1
    So at my gym, there's a cable machine that works like this: You pull on one cable and you lift x mass using y force. You pull the other and find the same thing. If, however, you pull on both, you're still only lifting x mass (you can see the weights) yet somehow each cable is still requiring y force. I actually held ~80lbs in both arms, and then released one. Intuitively, I should then be resisting 160lbs with one arm, which aside from being well beyond my ability, should have at least been noticeable. Yet I found no noticeable difference at all.
    I was just wondering how a system could be set up to allow for this kind of thing.
  2. jcsd
  3. Aug 2, 2013 #2


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    Science Advisor

    See the pulley on the right:


    The forces in the cables are equal, and stay the same no matter if you pull one of them or both.
    Last edited by a moderator: May 6, 2017
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