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Homework Help: Moment about a point problem

  1. Oct 5, 2009 #1
    1. The problem statement, all variables and given/known data

    I need to find the moment of A about D. We are given the Fg on the object to be 49N and the distance from A to D is 27.5cm. I've attached a drawing to help demonstrate.

    2. Relevant equations

    MD= rA/D X F

    3. The attempt at a solution

    I know the radius between A and D is 27.5cm but I'm not sure how to find out how much of the force is actually acting on D! I know I need to use sin or cos of a deduced angle (15 degrees or one relating to that). I'm not sure which trig operation or angle to use.

    Would anyone be able to shed some light on how to go about? Much thanks.

    Attached Files:

  2. jcsd
  3. Oct 5, 2009 #2


    User Avatar

    Staff: Mentor

    Welcome to the PF. The equation that you wrote is a vector equation, so you only need to write the vectors r and F, and take their cross product. The definition of the cross product includes the trig function that you need to use. Does that help?
  4. Oct 5, 2009 #3
    Muchly appreciated I understand right now. Simply break the radius and force vector into it's components and cross. I knew it was easier than I thought; my frustration was unjustified!

    Thanks man.
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