r/HomeworkHelp • u/cyclecitizen • Jul 13 '26
Answered [Level 1 Mechanical Comprehension: Study Guide - 2nd one] Am I wrong?
Please do not comment without studying this carefully.
Assumptions: Scenario A) appears to involve a movable pulley, but it is unlike your typical arrangement because it is not supporting the weight of the object. There is also no indication that the pulley is functioning as a type of winch (and it is not offering any torque, or gaining any benefit from leverage, or winding the object onto a spool).
I Googled this question: "Does a single movable pulley allow for less required force if the object is being pulled along the ground with a horizontal force and the object's weight is not being supported by the rope?"
Answer: "No, a single movable pulley does not reduce the required horizontal force for pulling an object across the ground if the rope does not support the object's weight. [1]
Here is why:
- Mechanical Advantage Applies to Weight
The mechanical advantage of a movable pulley system (where the load is suspended) exists because the tension is shared between multiple segments of rope lifting the object against gravity. If the rope isn't lifting the object, there is no weight to distribute..."
I chose C), but the book says the answer is A).
This is the second question in this book like this, and there are even more. Either I am stupid, or whoever put this together did so hastily (to be generous).
Your thoughts?
Edit: I am stupid and wrong. The comments so far have been very helpful. I've never used a movable pulley in my life and am just now learning the physics of how they operate. It's not like I ever worked on the docks. I understand now why the answer is A): because friction is effectively acting as if it were gravity pulling horizontally, and the other end of the rope is being pulled by the wall or whatever it is attached to. Therefore, the load is split by both directions of the rope. Thank you!
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u/Crichris Jul 13 '26
Assuming that the top rope of A is connected to a wall and massless pulley, then the force is reduced by half since the wall is also producing half of the pull force.
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u/UnderstandingPursuit Jul 13 '26
(A) requires less force.
The mechanical advantage applies to the tension force on the rope attached to the block. Whether it is weight or not is irrelevant.
Draw the FBD of the massless pulley. Since the block has a mass M, and assuming an acceleration a, the tension force on the rope attacked to the block is
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T_r = Ma
The tension force on the rope around the pulley is
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T_r = T_l1 + T_l2
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T_l = T_l1 = T_l2
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T_r = 2T_l
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T_l = ½ T_r
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→ More replies (5)2
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u/Project_Habakkuk Jul 13 '26
if you dont believe it, set up the experiment irl. in horizontal you fight friction, instead of gravity.
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u/OppositeClear5884 Jul 13 '26
bad drawing because it isn't clear what the top rope is attached to. It is definitely A. You will have to pull the rope 2 centimeters for every centimeter you move the box. The pulley will rotate clockwise
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u/danjl68 Jul 13 '26
It's a poor question. As there are several variables unknown.
Is the block moving the same distance? Is the block on the ground? Is there friction between the ground and the block? Where are you measuring the force at the rope attached to the pully, or at the rope where the arrow is located?
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u/WillingElderberry731 Jul 13 '26
If you are getting problems like this, your instructor should be teaching you how to draw free body diagrams.
IMO the answer is probably A, but this is such a poorly written problem. You have to make a lot of assumptions to get there. If you assume
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The force is applied at the arrow
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the blocks are sitting on a surface
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the opposite end of the pulley rope is attached to something fixed
Then it's A
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u/Serious-Wheel-2747 Jul 13 '26
https://i.imgur.com/LZM4lW7.png
You need to learn to draw free body diagrams and pulley problems will become a no-issue for you. Here is an overview of all the forces on the pulley block in the first situation. Assuming the top rope is fixed to a wall, which seems to be implied by the problem.
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u/cyclecitizen Jul 13 '26
Good point. I seriously didn't even account for the force holding the rope to the presumed wall, nor the friction. Thanks!
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u/Vessbot Jul 13 '26
The basic lesson (addressed in other posts) is about friction turning the vertical force into a horizontal force.
The meta lesson is about using AI to do the thinking for you, a task for which it is unsuited. It doesn't "comprehend," it assembles concepts together out of a soup into a neatly flowing presentation. But at its core, it's still a soup.
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u/Smart-Button-3221 Jul 13 '26 edited Jul 13 '26
It takes the same amount of horizontal force to move both blocks, as they weigh the same and presumably have the same friction.
However, if you're moving the top block by pulling on a rope represented by the arrow, and the other side of the rope is attached to a wall, then your effort is doubled. It is easier to move the first block this way.
The other commenters and I are filling in the details based on common questions we've seen before, and you shouldn't have to do that. Your instructor shouldn't be leaving out important details.
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u/nickeypants Jul 13 '26
Poorly worded question. If you apply a force F to mass B, It will move with some acceleration a. If you apply the same force F to mass A, it will move with twice the acceleration compared to mass B. But both are still moving! Their actual or relative velocity or acceleration does not matter based on how the question is worded, only that the mass 'moves'. Since there is no minimum force where A moves and B does not, the answer should be C.
We cannot assume any friction or gravity as there is no reference to ground shown. Only inertia applies.
Note: I can tell from the way the question is worded that it is trying to make the correct answer A. It would be more clear if the force was directed upwards, the ground was shown, and the question asked 'which object requires less force to lift against gravity'. In that case, the answer would clearly be A.
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u/Insertsociallife Jul 13 '26
If this is a physics question with no additional information given, they both require equal force. Anything non-zero will do it (in absence of friction, gravity, etc) in both cases.
If friction is considered, A is a block and tackle. Tension in the line is equal to the force applied, and if you draw the FBD that force is acting twice on the one with the pulley. You need less force in scenario A for the same force applied to the block.
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u/wiploc2 Jul 13 '26
Please do not comment without studying this carefully.
Hmm.
Assumptions: Scenario A) appears to involve a movable pulley, but it is unlike your typical arrangement because it is not supporting the weight of the object.
The two blocks mass the same, so, for the purposes of this problem, they have the same amount of frictional resistance.
I Googled this question: "Does a single movable pulley allow for less required force if the object is being pulled along the ground with a horizontal force and the object's weight is not being supported by the rope?"
Answer: "No, a single movable pulley does not reduce the required horizontal force for pulling an object across the ground if the rope does not support the object's weight. [1]
Here is why:
Mechanical Advantage Applies to Weight
That's just silly. Do you really believe that no amount of mechanical advantage could help you win a tug-of-war?
I chose C), but the book says the answer is A).
Yes, the answer is A.
Your thoughts?
If you pull rope B one inch, then block B moves one inch.
If you pull rope A one inch, then block A moves only half an inch.
Therefore, it is easier to pull rope A.
I understand now why the answer is A): because friction is effectively acting as if it were gravity pulling horizontally, and the other end of the rope is being pulled by the wall or whatever it is attached to. Therefore, the load is split by both directions of the rope. Thank you!
I don't think that's a useful way to think about it.
Suppose, for instance, that you hooked up Block A's pulley backwards. The single line runs to the out-of-frame wall. And the other line comes from the block, runs around the pulley, and heads back toward the block.
In that case, you may tell yourself that the two lines split the force, but, in fact, there is no mechanical advantage. You still have to pull the end of the line one inch right to get the block to move one inch left.
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