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Cross Product Confusion

  1. Nov 17, 2011 #1
    Hi all,

    Just having some trouble understanding a certain example of cross product. It's actually for physics, but figured it belongs in this forum.

    In the question I am supposed to cross (dl)y hat with (x1)x hat.

    So I go (0 dl 0) x (x1 0 0) = (-x1dl)z hat. But turns out the answer the computer looking for is the same but without the negative. Wouldn't that be the case if i crossed it backwards, i.e. (x1)x hat cross (dl) y hat ??
     
  2. jcsd
  3. Nov 17, 2011 #2
    Yes,
    [tex](0, dl, 0)\times{(}x_1, 0, 0)=-xdlk[/tex]
    and
    [tex](x_1, 0, 0)\times{(}0, dl, 0)=xdlk[/tex]
     
  4. Nov 17, 2011 #3
    Well then if that's the case I'm still confused. Because then my answer should be right.
     
  5. Nov 17, 2011 #4

    mathman

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    You need to write the question exactly as it presented, since the cross product depends on which comes first.
     
  6. Nov 17, 2011 #5
    Ok, I am thinking that there is an error in the system, because according to the Biot Savart Law it has a dl x r in the numerator, where r = (x1 0 0) and dl = (0 dl 0). So I computed that but yet my answer was wrong.
     
  7. Nov 18, 2011 #6

    mathman

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    Check the computer program specs. Presumably it will tell you how it interprets a cross product for the order in which the vectors are input.
     
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