Vector ( i-j = k) ?

  • #1

Homework Statement


I cant understand why the vector MA finally become (i+j+k) ?? is it beacuse of (i-j ) becomes k ??

Homework Equations




The Attempt at a Solution

 

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Answers and Replies

  • #2
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That's wrong!
[itex]
(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
[/itex]
 
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  • #3
haruspex
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That's wrong!
[itex]
(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
[/itex]
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.
 
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  • #4
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-j X i=k
That k and the other normal one from i x j=k, add up to 2k.
 

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