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The direction of R

  1. Nov 16, 2013 #1
    Dear friends, and science lovers,

    I am doing some statics problems with moment( cross-product).And I've been having some problems solving them.I am getting the result but in opposite sign. I mean I know we have to do R x F , but looks like I am taking R in the wrong a. For example we have to determine the moment of a force at A about point P.Should I find the vector from A to P or from P to A.Same thing in the couple forces, where we have to find the vector R between the couple forces and to multiply with one of the forces.I am getting that vector but I dont know with which force to multiply.If you can help me clear this thing on my mind I would be really thankfull.

    Kind regards,

    Astrit Imeri

    ME student
     
  2. jcsd
  3. Nov 16, 2013 #2

    jhae2.718

    User Avatar
    Gold Member

    Moments are taken about a point. In the expression ##\vec{M} = \vec{r} \times \vec{F}##, the vector ##\vec{r}## is from the point you are taking moments about to the point of application of the force ##\vec{F}##. The following graphics (borrowed from Wiki) should illustrate this:
    Torque_animation.gif
    512px-Torque%2C_position%2C_and_force.svg.png

    You can check your results using the right hand rule (assuming you're using a right-handed coordinate system!).
     
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