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Machines and reducing forces

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jgalt42

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So we know that:

Ramp: If you increase the distance, you'll reduce F

Lever: If you increase the lever arm, you''ll reduce F

But for the Pulley: What does the machine do to reduce F?
 

MedPR

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So we know that:

Ramp: If you increase the distance, you'll reduce F

Lever: If you increase the lever arm, you''ll reduce F

But for the Pulley: What does the machine do to reduce F?

The rope/string used in a pulley increases the distance, thereby reducing the force for a constant amount of work.

Take a look at the first pulley on the left.

Tackles.png


If you attached a mass to it, the weight of that mass would be, mg. In order to lift that mass, you would have to exert a force, F. If you draw a free body diagram, you will have 2T pointing up, and mg pointing down. Since the tension in the rope is equal at all points:

F=T
2T=mg
2F=mg
F=mg/2

If you were just trying to lift that same mass off the ground, your free body diagram would show F pointing up, and mg pointing down and you would have F=mg.
 
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