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Couple moments

  1. Jun 24, 2015 #1
    1. The problem statement, all variables and given/known data
    For F = 70 lb, compute the combined moment of the two forces about (a) point O, (b) point C, (c) point D. The moments are positive if counterclockwise, negative if clockwise.

    2. Relevant equations
    M = F d , M = r x F

    3. The attempt at a solution
    eb7827a6-ea06-4264-b498-46a8d2b9d9da_zpsfrvx7swv.jpg
     
  2. jcsd
  3. Jun 24, 2015 #2

    SteamKing

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    The force F is a vector, and your calculations treat it as if it were a scalar.

    Do you understand how to apply M = r x F ?
     
  4. Jun 24, 2015 #3
    would it be like M = r x F where r= displacement vector from A to 0?
     
  5. Jun 24, 2015 #4
    i tried but i must be making a mistake
    20150624_210536_zps6csfxhl7.jpg
     
  6. Jun 24, 2015 #5

    SteamKing

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    The moment arm is the vector rOB , which is measured from point O to point B. Your calculation shows the opposite.

    The moment arm should be measured from the reference point of the moment to the head of the force vector.
     
  7. Jun 24, 2015 #6
    So when using M = rF, the vector r is always measured from the point of moment to the to the head of the vector. so the would look like to vectors with the heads pointing toward eachother? And what about M = Fd? the distance line should always travel to the head of the vector? unless you negate the vector then the tail is ok?
     
  8. Jun 24, 2015 #7

    Drakkith

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