# Thread: How do I start this vector problem

1. ## How do I start this vector problem

and , find a unit vector with positive first coordinate orthogonal to both and .

I can do the calculation myself, i'm just not sure where to start

2. Originally Posted by Allie
and , find a unit vector with positive first coordinate orthogonal to both and .

I can do the calculation myself, i'm just not sure where to start
Start by taking the cross product of a and b.

3. I did do that is there another step? I seem to be missing omething
for the dot product i get
(10i+j+20k)-(20i+j-10k) which would give me -10i+10k. I know the j coefficient is 0 but where did i go wrong with the others?

4. Originally Posted by Allie
I did do that is there another step? I seem to be missing omething
for the dot product i get
(10i+j+20k)-(20i+j-10k) which would give me -10i+10k. I know the j coefficient is 0 but where did i go wrong with the others?
I said cross product, not dot product.

Are you aware there are two different way of multiplying two vectors? Have you been taught how to take the cross product of two vectors?

5. yes im aware and i did the cross product i don't know why i said dot.

6. Originally Posted by Allie
yes im aware and i did the cross product i don't know why i said dot.
So -10i+10k is perpendicular to a and b. Obviously any scalar multiple of -10i+10k is also perpendicular. So 10i - 10k is perpendicular (and satisfies the requirement of the i-component being positive).

Do you know how to get a unit vector in the direction of 10i - 10k ?

7. yes you have to divide it through by the magnitude thank you

8. Originally Posted by Allie
no
You know how to do a cross product but can't construct a unit vector!?

Divide 10i - 10k by its magnitude.