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Homework Help: Vector product

  1. Sep 16, 2007 #1
    ok, this is my first post here so i don't know what kind of questions i can ask, so i'll fire away...
    e is unit vector, i'm showing a = e(e*a) - e x (e x a), where * is the dot-product and x is the cross product, i keep getting 3a, not a. Is my algebra off or?...
     
  2. jcsd
  3. Sep 16, 2007 #2

    robphy

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    Can you show your work so we can see where you might be going wrong?
     
  4. Sep 16, 2007 #3
    well that's a lot of stuff, ok i'll try, these are simple matrix operations, i think my positive/negative alternating on row operations are ok when finding det a, but still, the rhs is where the problem must be because e(e*a)=a, right?? so

    a= c1x + c2y + c3z

    and taking both cross products, e x (e x a), because all scalars of e = 1 i get

    c1 + c2 + c3 - ((c2-c1) - (c1-c3))x = 3c1
    c1 + c2 + c3 - ((c3-c2) - (c2-c1))y = 3c2
    c1 + c2 + c3 - ((c1-c3) - (c3-c2))z = 3c3

    so is this not =3a ??

    why was this question moved? did i post it in the wrong place?
     
  5. Sep 16, 2007 #4
    Use vector identities instead.

    a x (b x c) = (a.c)b - (a.b)c
     
  6. Sep 16, 2007 #5
    thanks
     
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