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Homework Help: Find the net couple

  1. Aug 1, 2014 #1


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    1. The problem statement, all variables and given/known data
    If P=0, replace the two remaining couples with a single equivalent couple, specifying its magnitude and the direction of its axis.

    3. The attempt at a solution

    Position vector of B(rb)=0.3i
    Similarly, rd=0.16j+0.17k

    Then I tried to resolve the forces into its individual components. For 34N force applied at B, in vector form,

    [itex]34 \cos 28.07 \hat{i}-34 \sin 28.07 \hat{j}-34 \cos 45 \hat{k} [/itex]
    For the other force at B, I can write -18k.
    At D, [itex]-34 \cos 28.07 \hat{i}+34 \sin 28.07 \hat{j}+34 \cos 45 \hat{k} [/itex]

    Now I take moments of all the above forces about the origin which gives me
    1.13i + 7.51j

    the magnitude of which is given by [itex]\sqrt{1.13^2+7.51^2} = 7.59[/itex]

    which is incorrect according to answer key.

    Attached Files:

  2. jcsd
  3. Aug 1, 2014 #2


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    Are you sure angle DEF is 45 degrees? It doesn't look it from the figure.
  4. Aug 1, 2014 #3


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    You calculated the components of the 34 N force incorrectly. The 34 N forces are parallel to the vector connecting D and E or F and B. Find the direction of those vectors from the coordinates of the points. Multiply the magnitude of the force with the unit vector in the appropriate direction.

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