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Easiest way for two men to lift a pipe

by voyager221
Tags: easiest, lift, pipe
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voyager221
#1
May6-12, 11:51 PM
P: 19
If rope is wrapped around a pipe and two men are lifting it up why is it easier to lift when the rope is loose around the pipe (A) and harder when the rope is tied tightly to the pipe in a knot at both ends (B) ?

I presumed this was because on A the weight is less concentrated?
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haruspex
#2
May7-12, 03:43 AM
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I'm guessing a bit at the diagram...
Rope attached to one end of pipe, rises at angle theta to horizontal to first porter, proceeds horizontal to second porter, descends at angle theta to attach to other end of pipe. And the question is, why does a longer rope (greater theta) make it easier on the porters?
Pipe weight W, tension in rope T = W cosec(theta)/2.
To balance the tension locally, each porter has to pull at angle theta/2 to vertical with force F = 2T sin(theta/2).
Thus F = W sec(theta/2)/2. This is minimised when theta is 0.
So it doesn't support your observation.
Maybe I have the diagram wrong.


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