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Elementary mechanical advantage question

  1. Dec 2, 2012 #1
    I would be much obliged if the engineers on the forum could answer the following query regarding mechanical advantage. I don't require exact calculations or anything, just a quick yes or no, along with some simple KISS type explanation will be gratefully received.

    So, we have a situation in which these tribespeople have discovered a huge block of granite 4' x 3' in section and 8' long, and this block is sitting on the edge of a deep ravine.

    For reasons which are immaterial, they wish to rotate the block through 90 degrees so that it does not sit on the edge of the ravine any more.

    Their first attempt is as in the pic below.

    http://imageshack.us/a/img204/8213/stonehenge1.png

    This attempt is unsuccessful however, as there is obviously unsufficient leverage being exerted on the block.

    So, they decide to attach a long steel beam to the back of the block, and tie the rope to the top of the beam, as in pic#2 below.

    http://imageshack.us/a/img853/9914/stonehenge2.png

    This attempt proves more promising, but they still need more mechanical advantage. This advantage could obviously be achieved by increasing the height of the beam once more, but what I would like to know is whether adding a pulley wheel to the top of the beam and extending the beam downwards, and attaching the rope to the bottom of the beam would result in any increased mechanical advantage compared to pic#2.

    Or, would the mechanical advantage remain exactly the same ?

    This proposal is depicted below, in pic#3.

    http://imageshack.us/a/img20/1710/stonehenge3.png

    All comments gratefully welcomed.
     
    Last edited: Dec 2, 2012
  2. jcsd
  3. Dec 2, 2012 #2
    I am guessing the mechanical advantage would remain exactly the same for the 2nd and the 3rd design
     
  4. Dec 3, 2012 #3

    Mech_Engineer

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    The pulley does nothing if the rope is still fixed to the beam. If the rope was tied back behind the pullers however, it could in theory up to double the amount of force they could apply.
     
  5. Dec 4, 2012 #4
    Thanks for replies. So, just to clarify, there is no increase in mechanical advantage in figure 3 no matter how far the beam were to extend down below the surface ? (ignoring the weight of the beam itself btw)
     
  6. Dec 5, 2012 #5
    There is no increase in mechanical advantage in figure 3, as you are not increasing the length of the lever arm. The lever arm is PERPENDICULAR to the force applied. The extended beam in your diagram is parallel to the force, hence it does nothing at best, but realistically would make it even harder to move .
     
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