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Finding a vector perpendicular to a plane

  1. Sep 10, 2012 #1
    How did they know to do PQ cross PR instead of PR cross PQ?
    And why did they not choose RP cross RQ?

    Look at picture.
     

    Attached Files:

  2. jcsd
  3. Sep 10, 2012 #2

    Mark44

    Staff: Mentor

    Since any vector that is perpendicular to the plane will do, it doesn't matter if they calculate PQ X PR or PR X PQ, which will just point the opposite direction.
    If you cross any two vectors in the plane that aren't parallel, you'll get a third vector that is perpendicular to the first two.
     
  4. Sep 10, 2012 #3
    Another question:
    if I have 3 vectors u,v, and w
    If i am trying to find u cross v cross w do i cross u with v first then w or v cross w then u?
     
  5. Sep 10, 2012 #4

    Mark44

    Staff: Mentor

    I'm pretty sure they associate left-to-right, so u X v X w would be the same as (u X v) X w.
     
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