Pulleys - Why do they work?

Hagar

I think this is the right forum, please feel free to move if it isn't. Thanks.

We all know that pulleys work, depending on their configuration they can halve, quarter etc the effort needed to lift a weight. But exactly why do they work? After all the weight at the end of the rope hasn't changed, if anything the friction of the pulley should make it more difficult to lift.

Answers in layman's language if at all possible please.

Son Goku

Work is the amount of force exerted over a given distance.
Work = Force X Distance.

If you exert a force of A newtons over a distance of B meters, you do an amount of work A X B.
To lift a bag of cement, for instance requires let's say 10 units of work. So we have to have A X B = 10.

If we make B a large number, let's say 20 meters, A only has to be a small Force. This is how the pulley works, it gets away with using very little force, by having the force operate over a long distance. The distance being the length of the rope.

Howeve there is also a simpler, non-physics, fact that the human body is designed in such a way that we can effectively grab a rope and run with it, better than we can lift things with upper body strength.

mawk

Son Goku wrote:

Work is the amount of force exerted over a given distance.
Work = Force X Distance.

If you exert a force of A newtons over a distance of B meters, you do an amount of work A X B.
To lift a bag of cement, for instance requires let's say 10 units of work. So we have to have A X B = 10.

san is indeed very correct about the logic, its doing the exact same ammount of work over a longer distance so it takes less force at any one time. the problem therein is that you have to cover such a bigger distance so it takes longer. ie you use less power for a longer period. you dont pull as hard but do pull for longer.

since power= (force X distance)/ time

its the same reason why if you fall off a bike its better to slide and roll across the ground for a while untill friction stops you rather than hitting a tree. you do the exact same ammount of work stopping, just the magnitude of the force that exerts on you depends on how fast or slow you do it.

Hagar

I just know this going to sound dumb, and I'm not really dumb, honest.

Howeve there is also a simpler, non-physics, fact that the human body is designed in such a way that we can effectively grab a rope and run with it, better than we can lift things with upper body strength.

I can see how that's true. So far so good.

you dont pull as hard but do pull for longer

Right, I get the concept of doing easy work for a longer time being the same as doing hard work for a shorter time. The amount of work need is a fixed amount.
Does this mean that a pulley is essentially a suspended fulcrum?

Capt'n Midnight

yes.

If you have a beam with a weight on the end and you lift up a meter you would need to pull up a meter of rope.

If you were to attach a pulley at x half way down then you'd need to pull a meter of rope to lift point x up by half a meter. And the weight would lift up by a meter.



pivot pulley Weight

o___________x__________M

Hagar

Thanks for the repiles. It's amazing how we use simple things like a pulley but never question how they work.