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Problem with pulley video 
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#1
Apr1810, 12:35 PM

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Hello,
I am having trouble understanding an example shown on youtube regarding pulleys. http://www.youtube.com/watch?v=vSsK7Rfa3yA#t=3m0s In the example in the video above (I linked to the time when the example starts), the narrator uses an example where he pulls a rope with 5 Newtons for 2 meters, making a weight of 10 Newtons raise 1 meter. He is demonstrating mechanical advantage. http://en.wikipedia.org/wiki/File:Pulley1a.svg That is the basic setup, with W = 10N However my understanding is that if 5 Newtons is applied to the end of that rope, the system will be in equilibrium. In the picture above, the implication seems to be that if the end of the rope has W/2 applied to it, there is no movement. So how can the person in the video be pulling on that rope with a force of 5 Newtons and make the rope move at all? Thanks. 


#2
Apr1810, 02:00 PM

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Hello DocZaius!
but once it is moving, it can be kept moving at constant speed without any net force, so 5N is enough. 


#3
Apr1810, 10:07 PM

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Is that right? If this is correct, I feel it can be confusing and misleading to a student to be given X = 5N when X MUST be greater than 5N. 


#4
Apr1810, 10:30 PM

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Problem with pulley video



#5
Apr1910, 03:04 AM

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#6
Apr1910, 07:21 AM

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My interpretation of your previous reply is that once you attach a weight of 5N at the end of the rope, all it takes is a force greater than 0 pulling on the end of that rope (in addition, of course, to the 5N weigh you just attached) for the 10N weight to move up. If this interpretation is incorrect, please tell me where I go wrong. If this interpretation is correct, then it seems to me you can't move that 10N weight up with a force of exactly 5N. 


#7
Apr1910, 07:39 AM

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#8
Apr1910, 08:51 AM

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#9
Apr1910, 09:30 AM

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Btw, you still haven't quoted what the video actually says, that you disagree with. 


#10
Apr1910, 09:45 AM

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Imagine for the following picture that W=10 N and that a 5 N weight is attached at the end of the rope. No other forces  the weight attached is exactly 5N. This is the initial setup. What happens? Is there any movement? http://en.wikipedia.org/wiki/File:Pulley1a.svg I thought there wouldn't be, but your "can be moving" statement puts that into doubt again. Please note that the above scenario I just proposed is not the scenario in the video (since in the video there is movement). I am only asking about this scenario to see if I understand a more basic situation correctly (one in which the pulley system does not move). 


#11
Apr1910, 11:03 AM

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There will be no acceleration. In your original example, if the instructor is applying exactly 5N to the end of the rope, then the rope isn't accelerating. But it certainly can be moving. 


#12
Apr1910, 11:23 AM

P: 295

Then back to my problem with the video. I cannot quote something from the video as you request, since my problem lies with what is not said. The instructor never accounts for the reason the rope is initially moving. He never says the initial setup is moving (shouldn't we assume an initial setup is not moving if it is not explicitly stated otherwise?). He never says he initially pulls with a force greater than 5N (to accelerate it), to then go back to exactly 5N (to keep it moving at the same speed). One is left to wonder, why is the rope initially moving without any accounting for it? The reason I bring this up is that many students would assume a force of 5N was enough to accelerate it. They will watch the video and think "pulling that rope with 5N accelerated that rope since the instructor stated no other interaction made with the system" when as you say, 5N only keeps it moving at its speed. 


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