# Homework Help: Dot & cross products

1. Oct 26, 2007

### kingwinner

Q:Given non-zero vectors u and v, find a vector x such that both equalitites x dot u=|u|, and x cross u = v hold. What should be the mutual position of the given vectors u and v in space, so that the problem has a solution?

What I've got so far:

Setting x = au + b(u cross v)
I have that
a=1/|u| and b= +/- |v| / |u cross (u cross v)|

This answers the first part of the question, but I don't get the second part (i.e. What should be the mutual position of the given vectors u and v in space, so that the problem has a solution?)

Thanks for helping and explaining!

2. Oct 27, 2007

### kingwinner

This question looks hard and weird, but I am sure someone here knows the answer...

3. Oct 27, 2007

### HallsofIvy

I started to say this was impossible until I saw that second question! Just "any" u and v won't do. You know, I expect, that the cross product of two vectors is perpendicular to both. Okay, if x cross u= v, what must be true of u and v?

Last edited by a moderator: Oct 28, 2007
4. Oct 27, 2007

### nizi

using the second equation,
$$\left( { x \times u } \right) \times u = v \times u$$
①$$u \left( { x \cdot u } \right) - x \left( { u \cdot u } \right) = v \times u$$
using the first equation and developping
②$$x = \frac{1}{\left| u \right|} u - \frac{1}{\left| u \right|^{2}} v \times u$$
hence x is obtained.
then considering the following 3 eqations,
$$\left( { x \times u } \right) \cdot u = v \cdot u$$
$$\left( { x \times u } \right) \times v = v \times v$$
③$$\left( { x \times u } \right) \cdot v = v \cdot v$$
respectively
④$$0 = v \cdot u$$
⑤$$u \left( { x \cdot v } \right) - x \left( { v \cdot u } \right) = 0$$
l.h.s in ③ $$= x \cdot \left( { u \times v } \right)$$ so substituting ① into ③
⑥$$x \cdot \left( { - u \left( { x \cdot u } \right) + x \left( { u \cdot u } \right) } \right) = \left| v \right|^{2}$$
substituting ④ into ⑤
$$u \left( { x \cdot v } \right) = 0$$
i.e
⑦$$x \cdot v = 0$$ because $$u \neq 0$$
according to ④ and ⑦ we can say $$u$$ and $$v$$ are orthogonal and $$u$$ and $$x$$ are on the same plane perpendicular to $$v$$ and not parallel due to the given first equation.
further using ①, ⑥ we can also say the relationship of the norm of $$u$$, $$v$$ and $$x$$ as
$$\left| u \right|^{2} \left( { \left| x \right|^{2} - 1 } \right) = \left| v \right|^{2}$$

Last edited: Oct 27, 2007
5. Oct 28, 2007

### kingwinner

I forgot to type the hint that is attached to the orginal question:
HINT: seek the vector x in the form of a linear combination of u and (u cross v)

But I'm not sure how this will help...

6. Oct 28, 2007

### kingwinner

So v and u must be perpendicular, right?
Is that all?

Last edited: Oct 28, 2007
7. Oct 28, 2007

### HallsofIvy

Yes.

8. Oct 29, 2007

### kingwinner

But there is another condition that it must satisfy: x dot u=|u|, does this impose any other restrictions?