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M1 one moments how do i decide if the moment direction clockwise or anti clockwise? watch

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    (Original post by Chwirkytheappleboy)
    Pivots tend to be fixed points (unless you have a more dynamic system than you'd typically see at A level) so you assume that the rod is stationary and therefore the clockwise moments equal the anticlockwise moments.

    Considering the moments about A, there is a downwards force of 20N which is 1.5m away, and an upwards force of RcN which is 2m away (expressed as 1.5+0.5 because they're the measurements shown on the diagram). The downwards force creates a clockwise moment which must be equal to the anticlockwise moment produced by the upwards force. Hence the equality
    right i see now, the question was referring to the centre of mass as the pivot, although now that i think about it this wouldn't make any sense. So since the sum of the moments are 0  20 \bullet 1.5= 2 \bullet R_c as these are two opposing moments thanks, so where does the 1 meter come into this?
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    O i realise now B is also a pivot which is where the 1 meter comes in.
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    (Original post by Core)
    where does the 1 meter come into this?
    Dunno, I'd have to see everything that the book is saying about it. It's possible that they may go on to say that that section of rod has a mass and that it contributes to the moment, but I can't see anything that makes reference to it in what you've posted here.
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    B must have 0 reaction, which would leave me with many unknowns since the center of mass is between the points A and C.
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    Yeah it says i could choose points A, B or C. but that A and C are more simple to use to answer the question.
 
 
 
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Updated: April 11, 2011

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