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Homework Help: Vector ( i-j = k) ?

  1. Jan 10, 2015 #1
    1. The problem statement, all variables and given/known data
    I cant understand why the vector MA finally become (i+j+k) ?? is it beacuse of (i-j ) becomes k ??

    2. Relevant equations

    3. The attempt at a solution

    Attached Files:

    • kgk.png
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  2. jcsd
  3. Jan 10, 2015 #2


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    Gold Member

    That's wrong!
    (i-j)\times (i+j)=\underbrace{i\times i}_0+i \times j- j\times i-\underbrace{ j \times j}_0=i\times j+i\times j=2 i \times j=2 k
  4. Jan 10, 2015 #3


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    Science Advisor
    Homework Helper
    Gold Member

    It's wrong before that, even.
    ##\vec P = P(\vec j -\vec k)/\sqrt 2##
    Alternatively, by calculating the distance from A to the line of P the moment is seen to have magnitude ##aP\sqrt {\frac 32}##. I believe the correct answer is exactly half that shown.
  5. May 18, 2015 #4
    -j X i=k
    That k and the other normal one from i x j=k, add up to 2k.
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