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Direction of moment

  1. Sep 25, 2015 #1
    1. The problem statement, all variables and given/known data
    Why shouldn't the direction of moment for 1200n at A =clockwise? Why the book gave counterclockwise?

    2. Relevant equations


    3. The attempt at a solution
     

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  2. jcsd
  3. Sep 25, 2015 #2
    It depends if you are talking about the moment of the force (1200 N), which is turning clockwise or if you are looking at the reaction moment in the connection of the beam to the wall (MA), which has to turn counterclockwise to establish the equilibrium. Two moments of the same size in two different directions.

    EDIT: Sorry, I didn't look at the drawing well: The moment MA of course must be the sum of the moments produced by the force (1200 N) and the weight of the beam (both with clockwise direction).
     
    Last edited: Sep 25, 2015
  4. Sep 25, 2015 #3

    haruspex

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    Well, no, as you wrote in the first paragraph MA would be equal and opposite to the sum of those two moments, hence anticlockwise.
    Anyway, it really doesn't matter which way it is presumed to be. MA could be taken to be clockwise, and the equations written accordingly. It would then turn out to have a negative value.
     
  5. Sep 25, 2015 #4
    the correct
    the correct moment would be in clockwise direction , right ? the anticlockwise in merely the reaction of the moment produced which is in opposite direction , am i right ?
     
  6. Sep 25, 2015 #5
    That was what I meant to say in my EDIT. Sorry for the ambigous expression.
     
  7. Sep 25, 2015 #6

    haruspex

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    In the textbook extract you posted, MA is the reaction moment. Read it carefully. There is no error there.
     
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