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Homework Help: Representing two couples by a force F

  1. May 1, 2014 #1
    1. The problem statement, all variables and given/known data

    Sorry if I posted in the wrong section, this is actually an App Maths question, but anyway

    The two shafts are subjected to the
    torques (couples) shown.

    (a) If you represent the two couples by a force F acting
    at the origin O and a couple M, what are F and M?

    (b) What is the magnitude of the total moment exerted
    by the two couples?



    2. Relevant equations

    3. The attempt at a solution

    I do know what M is, long story but my final answer is 6.05Nm. I just don't get how to solve for F and I am completely stuck, any help ?
  2. jcsd
  3. May 1, 2014 #2

    And check your units.
    Last edited: May 1, 2014
  4. May 2, 2014 #3


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    I'm subscribing out of curiosity; could you fill in something under 2. and relate to us the long story ? I take it you added up the two pseudovectors ?
  5. May 2, 2014 #4
    Firstly I found the components of the moments by using simple trigonometry. I then summed them up and finally to get the magnitude I squared each component added them and took the square root
  6. May 2, 2014 #5


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    Do you have the dimensions of the cube ?

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  7. May 2, 2014 #6
    Dimensions of the cube are irrelevant.
  8. May 3, 2014 #7
    No, the dimensions are not given. I think the force is zero because the two couples each have two equal and opposite forces(by definition of a couple) that would cancel. Is that correct ?
  9. May 3, 2014 #8


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    I think you are correct. ... but then I'm the guy who asked about dimensions.
  10. May 3, 2014 #9
    I agree the resultant force is zero.
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