# Vector cross product

1. May 7, 2008

### Ry122

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

2. May 7, 2008

### Defennder

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?

3. May 7, 2008

### 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))

4. May 7, 2008

### D H

Staff Emeritus
This equation for the projection is

$$\mathbf w = \mathbf v \frac {\mathbf u \cdot \mathbf v}{v^2}$$

Note well: The cross product is not involved when you compute the projection this way.

5. May 7, 2008

### 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.