The cylinder moves downwards a distance x. How far up does the box move in that time?

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#1
Q (part a): http://prntscr.com/uephr4

I don't think there is a way to solve this question as I'm dealing with 3 unknowns: T2, x, & a. I tried using simultaneous equations but I end up with 6g = 0a.

My diagram: http://prntscr.com/uepq82

Since the box and the cylinder have the same acceleration they should both travel a distance of x if they're stop times are equal. So why is the distance for the box 2x?
Last edited by TSR360; 2 months ago
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2 months ago
#2
It should be a simple string length question. No forces are needed.
The string length is constant, if the left part and middle parts are longer (how much?), how much shorter is the right part?

edit - they don't have the same acceleration.
Last edited by mqb2766; 2 months ago
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#3
(Original post by mqb2766)
It should be a simple string length question. No forces are needed.
The string length is constant, if the left part and middle parts are longer (how much?), how much shorter is the right part?

edit - they don't have the same acceleration.
Wouldn't I need to know what proportion of the string extends extends from the pulley to the box & from the ceiling to the cylinder?
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2 months ago
#4
(Original post by mqb2766)
It should be a simple string length question. No forces are needed.
The string length is constant, if the left part and middle parts are longer (how much?), how much shorter is the right part?

edit - they don't have the same acceleration.
I'm not the OP and not looking for a solution, but this looks interesting as it's a bit trickier than the usual "two objects hanging over a pulley" questions!

I can see where the 2x distance has come from, but can I just check - is the OP's diagram correct apart from the assumption that the accelerations are equal?

So our assumption in solving this is that the tape's tension is constant throughout, and without further information we can only write the box's acceleration and the overall tension in terms of a. Am I thinking this out correctly?
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2 months ago
#5
(Original post by davros)
I'm not the OP and not looking for a solution, but this looks interesting as it's a bit trickier than the usual "two objects hanging over a pulley" questions!

I can see where the 2x distance has come from, but can I just check - is the OP's diagram correct apart from the assumption that the accelerations are equal?

So our assumption in solving this is that the tape's tension is constant throughout, and without further information we can only write the box's acceleration and the overall tension in terms of a. Am I thinking this out correctly?
The left object goes down by x. The left and middle substring must be x longer each. To the OP, you need differences, not proportions. They rob this from the right substring which must be 2x shorter.

Parts b&c bring tensions and accelerations into the probkem. It's not needed for a). Part c) says the tape tension is constant throughout?

Obviously this is the basis for pulleys, where the work is
Force*distance
So you use a pulley to apply a higher force over a shorter distance for the same work done. See pulley - mechanical advantage examples in
https://www.bbc.co.uk/bitesize/guide...6yc/revision/6
Last edited by mqb2766; 2 months ago
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2 months ago
#6
One of these days we might even get a block and tackle question on here - unless they're all in physics help.
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2 months ago
#7
This is a (simple, rearranged) gun tackle
https://en.m.wikipedia.org/wiki/Block_and_tackle
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2 months ago
#8
(Original post by mqb2766)
Part c) says the tape tension is constant throughout?
As memory serves, "smooth pulley" is the magic phrase that says "you can assume the tension is constant along the string".
(Original post by ghostwalker)
One of these days we might even get a block and tackle question on here - unless they're all in physics help.
I thought Step I, 2014, Q11 was quite a fun pulley question.
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2 months ago
#9
(Original post by DFranklin)
As memory serves, "smooth pulley" is the magic phrase that says "you can assume the tension is constant along the string".

I thought Step I, 2014, Q11 was quite a fun pulley question.
Of course, I was just emphasising that it was hinted at / stated in the question part, as well as the original smooth pulley statement.
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#10
(Original post by mqb2766)
The left object goes down by x. The left and middle substring must be x longer each. To the OP, you need differences, not proportions. They rob this from the right substring which must be 2x shorter.

Parts b&c bring tensions and accelerations into the probkem. It's not needed for a). Part c) says the tape tension is constant throughout?

Obviously this is the basis for pulleys, where the work is
Force*distance
So you use a pulley to apply a higher force over a shorter distance for the same work done. See pulley - mechanical advantage examples in
https://www.bbc.co.uk/bitesize/guide...6yc/revision/6
This info on pulleys wasn't covered at all in my textbook. Is it common knowledge or are they assuming I study physics?
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2 months ago
#11
(Original post by TSR360)
This info on pulleys wasn't covered at all in my textbook. Is it common knowledge or are they assuming I study physics?
That link was to gcse physics.
Tbh, it's the simplest such pulley and it's not unreasonable that you could apply the stuff you learnt on a level mechanics to a simple gcse-type problem. The question tries to walk you through the approach by getting you to think about distances, then accelerations, then tension. Rather than the other way round. So a bit unusual but not unreasonable.
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#12
(Original post by mqb2766)
That link was to gcse physics.
Tbh, it's the simplest such pulley and it's not unreasonable that you could apply the stuff you learnt on a level mechanics to a simple gcse-type problem. The question tries to walk you through the approach by getting you to think about distances, then accelerations, then tension. Rather than the other way round. So a bit unusual but not unreasonable.
I don't remember covering pulleys in that much depth in my GCSEs. I looked up the word in my old Edexcel CGP physics textbook and it only appears 5 or 6 times in total. The link you provided is for the AQA GCSE spec, so maybe that is why? Looks like I need to get the AQA CGP textbook now...lol
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2 months ago
#13
(Original post by TSR360)
I don't remember covering pulleys in that much depth in my GCSEs. I looked up the word in my old Edexcel CGP physics textbook and it only appears 5 or 6 times in total. The link you provided is for the AQA GCSE spec, so maybe that is why? Looks like I need to get the AQA CGP textbook now...lol
Can't give you a definitive answer, sorry. Tbh, I'd just concentrate on understanding why the tension is constant, hence the tensions pulling upwards is double for one mass. That's the main thing. The question gets you to "discover" this in part c).
Last edited by mqb2766; 2 months ago
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