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Math Help - Direction of Moments

  1. #1
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    Direction of Moments

    Just a quick question...

    If I get an answer of say... M = 220i + 70k (Nm) for a moment, and the answer in the back of the book is M = -220i - 70k (Nm), what would cause this?

    I'm using the cross product for calculating the moment. I'm positive my r vector has the correct signs, so it either has something to do with the unit vector or my force.

    When doing the problem, I needed to calculate the moment about the base of a pole due to the force exerted at the top of a pole by a cable.

    So I calculated the tension in the cable, and then used the unit vector to find the components of the force. I then found a vector from the base of the pole to the line of action of the force, and used the cross product to calculate the moment. I just can't figure out why their answer is negative's and mine is positive's... hmm...
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  2. #2
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    Ahh... I needed to reverse the signs of my unit vector. The original unit vector pointed towards the pole when I found the tension, but when finding the moment the tension is pointing away from the pole.

    Oops!
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  3. #3
    MHF Contributor kalagota's Avatar
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    Quote Originally Posted by Thomas View Post
    Just a quick question...

    If I get an answer of say... M = 220i + 70k (Nm) for a moment, and the answer in the back of the book is M = -220i - 70k (Nm), what would cause this?
    the difference is the direction of the vectors. can you state the whole problem..
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