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Thread: Block system question

  1. #1 Block system question 
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    http://dev.physicslab.org/Document.a...ockSystems.xml


    Because it is a system you have to add the two blocks mass together. But at the same time it makes no sense to me because 100N is being exerted on the top block in both situations, the hand pulls the buttom block with 100N and gravity pulls on the block causing a force on the rope of 100N so wouldn't the top block on both move at the same rate? I'm sure I'm wrong but that site's explanation doesnt answer it clearly enough for me.


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    The total weight of the two block system is 200N, but only half of that is being subjected to gravity. So only half of the force needed for it to accelerate at G is applied. So the net acceleration would be half a G. In the one block system, a resultant force is being applied equal to if the single block was freefalling, so it will accelerate at G. Did that make sense?


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    In other words, you can replace the two block system with a single block at the top with a weight of 200N, being pulled by someone's hand with a force of 100N. Ergo, half the acceleration.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

    "Gullibility kills" - Carl Sagan
    "All people know the same truth. Our lives consist of how we chose to distort it." - Harry Block
    "It is the mark of an educated mind to be able to entertain a thought without accepting it." - Aristotle
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  5. #4 Re: Block system question 
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    Quote Originally Posted by EV33
    http://dev.physicslab.org/Document.aspx?doctype=5&filename=Compilations_Next Time_TwoBlockSystems.xml


    Because it is a system you have to add the two blocks mass together. But at the same time it makes no sense to me because 100N is being exerted on the top block in both situations,
    No, not true. In the first situation, 100N would be exerted if Block A were firmly fixed to the surface it is resting on. Since it is accelerating, that is no longer the case.
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  6. #5  
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    I'm pretty sure I got it now. So is what your saying is in the two block system gravity is puling the buttom block at g but the top block is resisting with 100N so its half the acceleration of g? The one block one is just how it looks I don't really know how to explain but I think I see the difference.

    And thanks for the help you guys this problem was bothering me all day.
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    Quote Originally Posted by EV33
    So is what your saying is in the two block system gravity is puling the buttom block at g but the top block is resisting with 100N so its half the acceleration of g?
    Be careful when you say it is resisting with 100 N; that is 100 "Newtons" of mass, not force. Its mass is 100/g kilograms. I don't like the use of Newtons as a unit of mass. I think it is confusing you.

    The accelerating force is 100 newtons. The system mass is (200/g) kilograms. Therefore the system acceleration is force divided by mass which is (100/200) g = 0.5g. Working backwards from this, Block A mass is 100/g kilograms and its acceleration is the system acceleration, 0.5 g so the force on Block A is F = ma = (100/g)*0.5g = 50 newtons.
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