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Homework Help: Vector cross product

  1. May 7, 2008 #1
    What is the cross product of -i x i? Is it negative 1 or is still just 0?
  2. jcsd
  3. May 7, 2008 #2


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    Homework Helper

    Note that for any 2 vectors a,b: -a x b = -(a x b). This reduces the problem to -(i x i).

    Now what is the vector product of a vector with itself?
  4. May 7, 2008 #3
    well can you tell me how the projection of u on to v
    u=-i+2j and v=i+2j is
    v=(3/5)i+(6/5)j ?
    The answer i got was (4/5)i + (8/5)j
    i used the equation
  5. May 7, 2008 #4

    D H

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    This equation for the projection is

    [tex]\mathbf w = \mathbf v \frac {\mathbf u \cdot \mathbf v}{v^2}[/tex]

    Note well: The cross product is not involved when you compute the projection this way.
  6. May 7, 2008 #5
    That's the same equation that I gave. yeah i realized my mistake after posting, i should have said dot product, not cross product.
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