Atwood's Machine: Finding Tension at A & D

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Poetria
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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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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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