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Max weight that anyone can lift using over head pulley.

  1. Jan 30, 2012 #1
    Can a man whose weight is 50KG (but powerful arms) lift another man with 70 KG weight using an overhead pulley.

    I guess no since the light man is not clamped to the groud, his arm strength will not bear any significance and he can lift only the wieght(on other side of the pulley) equal to his own weight. Please comment.
     
  2. jcsd
  3. Jan 30, 2012 #2

    AlephZero

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    If the light man pulled harder than 70 kg, both men would leave the ground at the same time, but the only way to keep both men off the ground would be for the light man to keep pulling harder than 70kg and climb up the rope faster (and higher) than the heavy man was lifted.

    If the light man stopped climbing, the tension in the rope would reduce to below 70kg and the heavy man would fall back to the ground.

    If the light man pulled with between 50kg and 70kg, he would climb the rope and the heavy man would not move.

    If there is no friction at the pulley, it doesn't make any difference which man pulls on the rope, provided the tension in the rope is greater than 70 kg.
     
  4. Jan 30, 2012 #3
    Thanks for the reply..So effectively the man pulling the rope (50 KG) cannot lift the heavier man (70KG). He can just climb the rope at his side. If he doesnt the rope may snap at max due to tension but in no way can the heavy man come above ground.

    Hence Can it be assumed that the maximum load anyone can lift on other side of pulley(frictionless) should be less than his own weight despite his powerful arms.


    Only way to Lift the heavy man would be to clamp the lighter man to ground somehow and then he can utilize his powerful arms to exert more that 70 KG pull.

    Please comment ..
     
  5. Jan 30, 2012 #4

    AlephZero

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    That's not what I said. Read it more carefully.
     
  6. Jan 30, 2012 #5
    If he climbs fast enough, he can lift the heavier guy, as AlephZero said. It's a question of acceleration: if he accelerates up the rope, he can exert a downwards force on it that is greater than his own weight. In this case both men rise, the lighter one faster than the heavier one. So the lighter guy will get to the pulley first.

    If he wants to keep his feet on the ground, yes. Otherwise see above.

    That'll work.
     
  7. Feb 1, 2012 #6
    AlephZero & Michael C .....Thanks a ton..I think this clarifies my doubts. :)
     
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