# Question about cross and dot product

leonne

## Homework Statement

There are two points p1 pointing up p2 pointing right

## The Attempt at a Solution

I need to find $$\stackrel{\rightarrow}{p1}$$ * r^

$$\stackrel{\rightarrow}{p2}$$ X p1^

$$\stackrel{\rightarrow}{p2}$$* r^

they got $$\stackrel{->}{p1}$$ * r^ =0 why is that? i know if they are perpendicual than they =0 but not sure where r^ is pointing

$$\stackrel{->}{p2}$$ X p1^ =p2 well i know when 2 vectors are paralle the = 0 so i am guess when they are perpendicular they are p2 , but only one is a vector not sure if that matters.

$$\stackrel{->}{p2}$$* r^ = p well i am guessing from this, that r^ is parallel with p2. that's why with p1 its 0, its perpendicular, but how do we find out which way r^ is pointing?

thxs

Mentor

## Homework Statement

There are two points p1 pointing up p2 pointing right
What do you mean that one "points up" and the other "points right"? A point doesn't have direction. Do you mean vectors?

## The Attempt at a Solution

I need to find $$\stackrel{\rightarrow}{p1}$$ * r^
What is r? Is r^ supposed to be a unit vector?
$$\stackrel{\rightarrow}{p2}$$ X p1^

$$\stackrel{\rightarrow}{p2}$$* r^

they got $$\stackrel{->}{p1}$$ * r^ =0 why is that? i know if they are perpendicual than they =0 but not sure where r^ is pointing
If two vectors are perpendicular, their dot product is zero. The vectors themselves are not necessarily zero vectors.
$$\stackrel{->}{p2}$$ X p1^ =p2 well i know when 2 vectors are paralle the = 0 so i am guess when they are perpendicular they are p2 , but only one is a vector not sure if that matters.
If two vectors are parallel, then one is a scalar multiple of the other. Also, their cross product is the zero vector.
$$\stackrel{->}{p2}$$* r^ = p well i am guessing from this, that r^ is parallel with p2. that's why with p1 its 0, its perpendicular, but how do we find out which way r^ is pointing?
Not much of what you wrote makes sense. Please include all of the given information for this problem.