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Question on pulleys force and rope pulling

  • #1

Homework Statement:

It is a picture.

Relevant Equations:

Force= Mass*acceleration. Mechanical advantage.
question34.jpg


I have a question about the following 33 and 34. For number 33 I think the answer is B because you have to life up against the force of gravity. While with A you have gravity helping you out. The correct answer is A but I am not sure how.

for 34 I think B because you have an extra pulley so you need less force but more distance.
 

Answers and Replies

  • #2
PeroK
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Homework Statement:: It is a picture.
Relevant Equations:: Force= Mass*acceleration. Mechanical advantage.

I have a question about the following 33 and 34. For number 33 I think the answer is B because you have to life up against the force of gravity. While with A you have gravity helping you out. The correct answer is A but I am not sure how.

for 34 I think B because you have an extra pulley so you need less force but more distance.
For 33, what would be the tension in the string/rope in each case if it was fastened to the floor or ceiling?
 
  • #3
I think rope B in question 33would have half the tension if it was anchored to the ceiling. But A has the pulley helping.
 
  • #4
PeroK
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I think rope B in question 33would have half the tension if it was anchored to the ceiling. But A has the pulley helping.
I like the first sentence. But, what do you mean by "A has the pulley helping"?
 
  • #5
I think I mean gravity would help because you are pulling down.
 
  • #6
jbriggs444
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I think I mean gravity would help because you are pulling down.
If I understand the reasoning correctly, since gravity is pulling your hands downward, your arms need not work so hard to move your hands against the rope.

However, this is not what the question is getting at. Ignore your hands. What tension is required on the rope to lift weight in situation A? What tension is required on the rope to lift the weight in situation B?

I can vaguely remember a decades-old intuition much like the one you are using here. It feels like it should be easier to pull down on a rope than to pull up. And yes, it is indeed easier to do a pull up than to do a curl. But we are not being asked about subjective ease. We are being asked about objective force.
 
  • #7
PeroK
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I think I mean gravity would help because you are pulling down.
Are you sure it works like that?

Physically, in terms of biomechanics, it's better to pull down where you can use your body weight. But, that's just a better way for you to generate a force; not that you physically need less force.
 
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  • #8
I think I get in B you have the taking ceiling subtracting away some of tension on the rope. So you only pull with half the force.
 
  • #9
PeroK
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I think I get in B you have the taking ceiling subtracting away some of tension on the rope. So you only pull with half the force.
Yes, it's a pity you can't just set this up as an experiment and see for yourself.
 

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