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Homework Help: Find the magnitude of the force

  1. May 19, 2007 #1
    A wrench 30cm long lies along the positive y-axis and grips a bolt in the origin. A force is applied in the
    direction < 0; 3; -4 > at the end of the wrench. Find the magnitude of the force needed to supply 100J of
    torque to the bolt.

    how can i solve it?

    r=<0,.3,0>
    F=<0,3,4>
    T= r X F =100
    then i solved it by finding the det = 100

    |i j k|
    |0 .3 0| = 100
    |0 3x -4x|

    then x = 83.333 N

    but the answer is wrong Why???
     
  2. jcsd
  3. May 19, 2007 #2

    malawi_glenn

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    Science Advisor
    Homework Helper

    you get x = 83.333

    but the magniture of the force is sqrt((3x)^2 + (-4x)^2 ) = 416.7 N

    magnitude of vector v=(a,b,c) is sqrt(aa + bb+ cc)
     
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