Atwood's Machine: Finding Tension at A & D

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The discussion revolves around calculating the tension in a rope passing over a fixed pipe, specifically at points A and D, with a focus on the tension ratio between points B and C. Initial calculations suggested tensions of 2 at A and 1 at D, but these were deemed incorrect due to the lack of friction mentioned in the problem. Participants noted that without friction, the system would not be static, leading to different tension values at B and C. The problem's title, "Wrapping friction," implies that friction should be considered, yet it was not addressed in the initial description. Ultimately, the tension questions at points A and D were recognized as trick questions, highlighting the need for clarification in the problem setup.
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Homework Statement



A rope of uniform mass density is passed above the top of a fixed pipe of circular cross section.
Points B and C - the points where the rope loses contact with the pipe.
Points A and D - at the ends of the rope
The length of the segment AB = 2d
The length of the segment CD = d.

The magnitude of the force of tension at the point A - ?
The magnitude of the force of tension at the point D - ?

I have calculated the ratio: tension at the point B over tension at the point C = 2

The Attempt at a Solution


[/B]
I thought tension at A = 2
tension at D = 1

Well, this is wrong. I am missing something.
It is somewhat similar to a simple Atwood machine, isn't it?
 

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There is no tension at the ends of the rope.
 
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Poetria said:
tension at the point B over tension at the point C = 2
As .Scott notes, there is no cause for tension at the rope ends, so I am not sure whether this is a trick question or you have misquoted it and actually want the tension ratio between B and C.
The description you give says nothing about friction. If there is no friction then there will surely be acceleration, so the tensions at B and C won't be the same as for a static arrangement. However, the image title mentions friction.
Please clarify.
 
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haruspex said:
As .Scott notes, there is no cause for tension at the rope ends, so I am not sure whether this is a trick question or you have misquoted it and actually want the tension ratio between B and C.
The description you give says nothing about friction. If there is no friction then there will surely be acceleration, so the tensions at B and C won't be the same as for a static arrangement. However, the image title mentions friction.
Please clarify.

Silly me! I got it.
"Wrapping friction" is a title of this problem. And there is no mention of friction afterwards.

There are three questions:
- about the magnitude of tension at the point A
- about the magnitude of tension at the point D
- about the ratio

So I see the first two are trick questions.
 
Poetria said:
So I see the first two are trick questions.
Then the last question is also... :smile:
 
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