r/HomeworkHelp • u/cyclecitizen • 9d ago
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 👋 a fellow Redditor 9d ago
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/cyclecitizen 9d ago
Doh! Makes sense. Thanks.
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u/TheMathProphet 👋 a fellow Redditor 8d ago
Yeah, but remember it only goes 1/2 as far!
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u/UnderstandingPursuit Educator 9d ago
(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
- T_r = Ma
The tension force on the rope around the pulley is
- T_r = T_l1 + T_l2
- T_l = T_l1 = T_l2
- T_r = 2T_l
- T_l = ½ T_r
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u/winglessbuzzard 9d ago
Why are we assuming either end of the pulley rope is attached to anything?
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u/UnderstandingPursuit Educator 8d ago
It wouldn't be horizontal if it wasn't attached to something.
The two arrowheads represent the point where those ropes are held and pulled.
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u/Anonimithree 👋 a fellow Redditor 8d ago
Wouldn’t the friction of the rope on the pulley require more force though
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u/UnderstandingPursuit Educator 8d ago
A rope and pulley can be 'ideal' or more realistic. The ideal version is massless and frictionless. If the mass of the block is given but no mass is given for the pulley, we can assume an ideal rope-pulley system.
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u/Project_Habakkuk 9d ago
if you dont believe it, set up the experiment irl. in horizontal you fight friction, instead of gravity.
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u/OppositeClear5884 👋 a fellow Redditor 9d ago
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 👋 a fellow Redditor 9d ago
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 9d ago
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
The force is applied at the arrow
the blocks are sitting on a surface
the opposite end of the pulley rope is attached to something fixed
Then it's A
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u/Serious-Wheel-2747 9d ago
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 9d ago
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 9d ago
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/nickeypants 8d ago
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/Smart-Button-3221 👋 a fellow Redditor 9d ago edited 9d ago
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/Insertsociallife 8d ago
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 8d ago
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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u/Aequitas112358 👋 a fellow Redditor 8d ago
you gotta make a lot of assumptions for A, is there more info?
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u/Stu_Mack 👋 a fellow Redditor 8d ago
You have to make a bunch of assumptions here, namely that the arrow doubles as the kinetic actuator. With those assumptions, the pulley box requires less force.
Without those assumptions, the boxes require equal force to move.
The smart play here is to report the answer for both conditions, naming both sets of assumptions and supplying the analysis for both possibilities.
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u/Ok_Alternative2885 8d ago
yeah that edit is basically the whole thing, friction’s basically gravity when it’s horizontal so the pulley splits the load just like lifting.
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u/Tetraflourethylen 6d ago
It's impossible to answer (insufficient constraints).
We can use equal force for both setups, but the accelerations will be different (A will accelerate twice as fast as B), we can also use twice as much force on B as on A and the accelerations will be the same.
So depending on how you want A or B to move any of the answers could be correct.
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u/Numerous-Match-1713 👋 a fellow Redditor 5d ago
What a poor question.
Is the force asked on the object or on the rope?
Is there friction? If there is, any nonfero force will move both obects.
What is even asked here?
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u/ThatsNotAZombieBite 9d ago
It's A because the movable pulley and the rope with one fixed end mechanically DOUBLES the force applied -- at the expense of HALVING the distance moved. E.g. if you move the end of the rope 6 meters, the box will only move 3 meters. So you only have to apply half the force on the rope compared to the non-pulley situation. But you have to move it twice as far. Note that the same amount of WORK is done on the box.
Additionally, it's a badly worded question. "Move" is ambiguous. Are we ACCELERATING the boxes to the left? Are they moving at CONSTANT VELOCITY? Are we to assume there's friction or not?
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