1. May 7, 2008 #1

   Ry122

   

   What is the cross product of -i x i? Is it negative 1 or is still just 0?

    

   Ry122, May 7, 2008
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3. May 7, 2008 #2

   Defennder

   

   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?

    

   Defennder, May 7, 2008
4. May 7, 2008 #3

   Ry122

   

   well can you tell me how the projection of u on to v
   where
   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
   w=v.((u.v)/(modulusv^2))

    

   Ry122, May 7, 2008
5. May 7, 2008 #4

   D H

   

   Staff Emeritus

   Science Advisor

   Insights Author

   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.

    

   D H, May 7, 2008
6. May 7, 2008 #5

   Ry122

   

   That's the same equation that I gave. yeah i realized my mistake after posting, i should have said dot product, not cross product.

    

   Ry122, May 7, 2008

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