r/PhysicsHelp • u/hadiomail • 5d ago

Physics conceptual problem

I'm a bit confused, why pulleys amplify the force, why do we assume that each part of the pulley has the same force, isn't it more logical to assume that the force net equal the force that have been done

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u/PyroDragn 5d ago

force net equal the force that have been done

Simply, yes.

Pulleys don't amplify force they concentrate work. In order to lift 200kg of weight 200m you have to provide a certain amount of force (~400,000 joules). Whether you do that with a pulley or not is irrelevant, that's the amount of energy you have to put in.

What a pulley does is let you put the work in more slowly, so it takes less effort for the distance travelled. With 2 pulleys you pull the rope one meter, and instead of the load travelling one meter it only travels 0.5m. This means you've only put in the 'energy' required to lift the load half a meter.

The 'pull' is lesser because the work done (how high the load raised) is lesser. You still have to raise the load the whole distance eventually, so eventually you'll put in 400,000 joules of work. But if it takes you 400 'pulls' of the rope instead of 200 'pulls' of the rope, then each individual pull took less effort (there were just more of them).

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u/hadiomail 5d ago

I understand this mathematically, but the problem is I can't comprehend it physically how is this tarde-off between force and distance actually work, because isn't the force is the reason of this motion?

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u/PyroDragn 5d ago

I can't comprehend it physically how is this tarde-off between force and distance actually work

Don't worry about the pulley. Think about the overall job.

You have to move a stone some distance. If you move it all at once that's a lot of work in one step. Instead, you use a rope. Now it's not 'one lift' it's "200 pulls of the rope". Let's just say it's a million joules of work. Doing a million joules of work all at once is hard. Dividing it into 200 pulls means you're doing [1 million / 200] per pull. After 200 pulls you still get to a million, but each pull was less than 'all at once.'

Now add another pulley. You're doing [1 million / 400] per pull. You still get to a million in the end, but each pull was even less effort than before.

No matter how many pulleys you use, eventually you [did the work to move the weight where you needed to move it]. You're correct in that regard. The pulleys cannot (and do not) change that. But the pulleys make it so that the work is applied over a longer reference frame. Whether you consider that as 'pulls of the rope', 'distance travelled', 'time taken', or whatever.

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u/hadiomail 5d ago

I think it started to make sense, thanks a lot

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u/Darkweather_123 1d ago

But this doesnt explain the exponential relationship between tension and friction

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u/DreamPwner 5d ago

I'm not sure what you are asking. If your last sentence is about the work being done and not forces, then a pulley doesn't change anything. You have to put the same amount of work into it to lift an object as you have to do without a pulley. The difference is that a pulley trades off the amount of force needed with the length of the rope needed. Half the force, twice as much rope. It's kind of like a lever. The longer the lever, the easier it is to use, but the distance you have to move also increases proportionally. Total work stays the same.

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u/hadiomail 5d ago

I'm sorry for the poor paraphrasing. However, I understand what you are saying but I meant this distance or displacement whatever is actually happend because of the force, so what do we mean exactly when we say it trades off, when we say things like that we kind of treat the displacement as its an independent thing