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M1 finding the tension, in a string using moments?

http://www.youtube.com/watch?v=m5uh8NNpg6E&feature=relmfu if you skip to 1:40,

My question is why does the fact that the tension passes through A mean that it does not have any affect? Is it something to do with the parallel rule for moments, also could someone explain that to me as well please.

Here it is the parallel rule(i don't know the actual name for it so im just calling that)
The moment of a parallel component force is 0.
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Ziltoid
Original post by Core
http://www.youtube.com/watch?v=m5uh8NNpg6E&feature=relmfu if you skip to 1:40,

My question is why does the fact that the tension passes through A mean that it does not have any affect? Is it something to do with the parallel rule for moments, also could someone explain that to me as well please.

Here it is the parallel rule(i don't know the actual name for it so im just calling that)
The moment of a parallel component force is 0.


A moment about a point, A, is equal to a force multiplied by the distance between the point A and the line of action of the force (in sense perpendicular to that force).

If this distance is 0, i.e. the force acts through the point, then the force multiplied by the distance is zero, and the moment is zero.
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Core
OP
6
Original post by Ziltoid
A moment about a point, A, is equal to a force multiplied by the distance between the point A and the line of action of the force (in sense perpendicular to that force).

If this distance is 0, i.e. the force acts through the point, then the force multiplied by the distance is zero, and the moment is zero.


So as there us no perpendicular distance there is no moment ,is that correct?
Reply 3
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Ziltoid
Original post by Core
So as there us no perpendicular distance there is no moment ,is that correct?


Precisely.
Reply 4
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Core
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6
Original post by Ziltoid
Precisely.


Much appreciated thank you.

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