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Homework Help: Pulley Conceptual Question

  1. May 8, 2014 #1
    I am having trouble understanding the distribution of forces present within a pulley system; namely, how the number of sections of rope that supports an item manages to decrease the force necessary to move it.

    For example, look at this picture


    I am aware that system A only requires 5 kg force to elevate while system B requires a 10 kg force; that being said, I do not understand why that is true.


    In this picture, it would appear that both weights are supported by the same number of rope-sections, yet B requires less force to elevate.
  2. jcsd
  3. May 8, 2014 #2

    Doc Al

    User Avatar

    Staff: Mentor

    The amount of force that the ropes exert on the mass depends on the tension in the rope and how many rope segments are pulling up on the mass. In B there is only one segment pulling up on the mass, thus the tension in the rope must equal the weight. In A there are two segments pulling up on the mass, so the amount of tension required is less.

    If you draw an imaginary box around the mass (including the attached pulley in B), you'll see that B has two segments pulling up on the mass.
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