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Grade 12th PassMechanics

A system of frictionless pulleys P1 and P2 are shown in figure. Neglecting the weights of the pulleys, the minimum pull needed at A to lift a load W will be
(1) W/2
(2) W/3
(3) W
(4) Below W/3
Pls explain stepwise and in detail.

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Profile image of Subhadip Dutta
8 Years agoGrade 12th Pass
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1 Answer

Profile image of Utsav Basu
ApprovedApproved Tutor Answer8 Years ago
The min force required to pull the weight up is actually the min force required to prevent the weight from falling. Thus, to simply solve this question we can solve it backwards. 
 
So, if we assume that the weight is not falling, a force equal to its weight must act upon it, which is W. This force is provided by the tention of the string wrapped around pulley P1, and is divided equally upon its two strands. Thus, the tension on the right strand = W/2
 
Now, notice that this force is actually provided by the pull at A. 
Thus, the min pull required to pull the weight up = W/2
Option (1) is correct.