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Math Help - vector HELP..

  1. #1
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    vector HELP..

    A force is given in the vector form, \hat{F}=3\hat{i}+4\hat{j}+\hat{k} passes thru the point P whose coordinates are [1,2,2] in a cartesian coordinate system and \hat{i},\hat{j},\hat{k} are the unit vectors in the x,y and z directions respectively. Q is a point whose coordinates are [4,3,6].

    i)Find the vector QP
    ii)find the maqnitude the moment of the force \hat{F} about the point Q.
    iii)Find the unit vector in the direction of the moment...

    If its possible to sketch a diagram too would be great...I just cant understand VECTORS!
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  2. #2
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    First part

    If you think about that \hat{F} act on P to go to T (end point of the vector) like:

    T = \hat{F} + P

    you see that:

    \hat{F} = P - T

    each coordinate is the coefficient of each of the fundamental vectors \hat{i},\hat{j},\hat{k}.

    In this execise:

    i) \hat{QP} = 3\hat{i}+\hat{j}+4\hat{k}

    I must go now... later i'll complete it.
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  3. #3
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    hm...interested to know how to solve this question. any help?


    And why isn't QP= OP-OQ= [ -3, -1, -4 ]?
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  4. #4
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    i would most probably used position vector to solve the 1st question:
    OQ = (4,3,6)
    OP = (1,2,2)

    QP = QO + OP
    = (-4,-3,-6) + (1,2,2)
    = (-3,-1,-4)
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  5. #5
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    so we have the same ans for part 1. what about the other two parts?

    is it:

    |Moment| = | F x QP |
    = | -15i+9j+9k |
    = sqrt 387

    on the right track?

    than for last part:

    unit vector in the direction of moment==> unit vector parallel with moment==> unit vector of moment.

    which is -15i + 9j +9k?

    is this the way?
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