Need help with pulley problem

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Francesenerf
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Joined: Wed Aug 22, 2018 2:42 pm

Need help with pulley problem

Post by Francesenerf » Wed Aug 22, 2018 4:55 pm

A loaded pail is attached to a rope that passes around and overhead pulley and is tied to a ring on the floor. Linda, a construction worker, plans to untie the rope from the ring, pull on the rope to lift the pail 1 m higher, and then re-tie the rope. Linda weighs 800 N and is capable of lifting twice her weight, 1600 N. The loaded pail weighs 1200 N.

What, if anything, is wrong with Linda s plan? Explain how to correct it or if the plan will work. Explain why?

I don't think she can lift it one meter then retie ut. is it because her applied force is less than the weight of the mass and pulley Is that the correct result.Please help me to proceed!I will be thankful for any help.

jeff
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Posts: 38
Joined: Mon Mar 05, 2018 11:54 pm

Re: Need help with pulley problem

Post by jeff » Wed Aug 22, 2018 5:51 pm

Hi Francesenerf,
Francesenerf wrote:
Wed Aug 22, 2018 4:55 pm
A loaded barrel is attached to a rope that passes around and overhead pulley and is tied to a ring on the floor. Linda, a construction worker, plans to untie the rope from the ring, pull on the rope to lift the barrel 1 m higher, and then re-tie the rope. Linda weighs 800 N and is capable of lifting twice her weight, 1600 N. The loaded barrel weighs 1200 N.

What, if anything, is wrong with Linda s plan? Explain how to correct it or if the plan will work. Explain why?

I don't think she can lift it one meter then retie ut. is it because her applied force is less than the weight of the mass and pulley Is that the correct result.Please help me to proceed!I will be thankful for any help.
Yes, you're correct that she will not be able to lift it with the rope, even though she is strong enough. (We have to assume she is simply standing on the floor, without being attached to anything.) However, that is not because of her applied force. It's because of the forces acting on her. It goes like this:

With the rope tied, the barrel is being pulled down by gravity with a 1200 N force, and so the rope tension has to be 1200 N to hold it in place. The rope is tied to a ring that is attached firmly to the floor, so it can provide the force needed to hold it in place.

When the rope is untied, and she is holding on to it, there is a weight force of 1200 N on one side of the pulley, and a weight force of 800 N on the other. That imbalance will cause the barrel to accelerate downwards (and she will accelerate upwards into the air if she keeps holding onto the rope!). The actual value of the tension will be between 1200 and 800 N and can be found by using Newton's law, if we needed it.

So that's the problem! The tension in the rope will be pulling upwards on her more than what she weighs, and so she'll be lifted right off the ground. Can you think of some way to arrange things so that she would be able to make this work?

Francesenerf
Posts: 3
Joined: Wed Aug 22, 2018 2:42 pm

Re: Need help with pulley problem

Post by Francesenerf » Wed Aug 22, 2018 6:20 pm

ok it falls to the ground and doesn't go up. makes sense

but I can't see how she could lift the barrel then. Can she tie herself to the ring or something like that?

jeff
Site Admin
Posts: 38
Joined: Mon Mar 05, 2018 11:54 pm

Re: Need help with pulley problem

Post by jeff » Wed Aug 22, 2018 6:33 pm

Francesenerf wrote:
Wed Aug 22, 2018 6:20 pm
ok it falls to the ground and doesn't go up. makes sense

but I can't see how she could lift the barrel then. Can she tie herself to the ring or something like that?
Yes, that could work. The important thing is that she be attached to something, so she could actually use yer strength to lift. As another possibility, there might be a short rail along the floor that she could slip her feet underneath that would keep her from being lifted off the ground. Then she could pull down on the rope with her maximum strength and lift the barrel.

Francesenerf
Posts: 3
Joined: Wed Aug 22, 2018 2:42 pm

Re: Need help with pulley problem

Post by Francesenerf » Wed Aug 22, 2018 6:43 pm

jeff wrote:
Wed Aug 22, 2018 6:33 pm
Francesenerf wrote:
Wed Aug 22, 2018 6:20 pm
ok it falls to the ground and doesn't go up. makes sense

but I can't see how she could lift the barrel then. Can she tie herself to the ring or something like that?
Yes, that could work. The important thing is that she be attached to something, so she could actually use yer strength to lift. As another possibility, there might be a short rail along the floor that she could slip her feet underneath that would keep her from being lifted off the ground. Then she could pull down on the rope with her maximum strength and lift the barrel.
ok makes sense, thanks for the help!

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