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Homework Help: Increasing the number of pulleys

  1. Jan 25, 2014 #1
    1. The problem statement, all variables and given/known data
    I'm trying to do a lab and none of my data is making sense. It's making it very difficult for me to draw conclusions and make sense of the post-lab questions with the data I have.

    Here are the questions I am struggling with:
    1. What happens to the force exerted as mechanical advantage gets larger?
    2. How does increasing the number of pulleys affect the ideal mechanical advantage and efficiency of a pulley system?
    3. How does increasing the load affect the ideal mechanical advantage and efficiency of a pulley system?

    2. Relevant equations

    IMA = de/dr

    MA = Fr/Fe

    Efficiency = (MA/IMA) x 100

    3. The attempt at a solution
    This is what I am assuming but I don't really know.

    1. I think the force gets smaller because the fr stays the same so to get a bigger MA you divide by a smaller number, plus the pulley makes it easier so you use less force. Would this be correct?

    2.I assumed that increasing the number of pulleys will increase the IMA. But due to friction it will decrease the efficiency. The more pulleys that are turning, the more friction there is, and the less efficient the system will be. Would this be correct?

    3. Increasing the load will add to the friction, thus reducing the efficiency of the system. The ideal mechanical advantage won't change since the IMA neglects friction. Does this make sense?

    Thanks for your help everyone!
  2. jcsd
  3. Jan 25, 2014 #2


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    Science Advisor
    Homework Helper
    Gold Member

    I don't like that the question refers to the force exerted. The force exerted is whatever it happens to be; that may be insufficient to move the load or far more than sufficient. Assuming it means the force needed, I agree with your answer.
    The question doesn't mean anything without the context of an actual arrangement of pulleys. A pulley could be added in a way that reduces MA. Other than that, I agree with your answer.
    Increasing the load will add to the friction, yes, but efficiency is about ratios. What will the affect be on the ratio of work done by the applied force to work done on the load?
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