# Velocity of fluid through a point on a plane

## Homework Statement

Fluid flows with velocity 2i - 3j m/s at point P having coordinates (1,2,4). Consider a plane through P which is normal to the vector b=-i+2k. What is the speed at which the fluid passes through the plane?

## The Attempt at a Solution

Should I do the dot product of the position vector P=[1,2,4] and b vector, then multiply this by unit vector that is in the direction of the b vector, and then dot the result with the velocity vector?

thats what i did but im getting a negative velocity

Last edited:

tiny-tim
Homework Helper
hi racnna! Fluid flows with velocity 2i - 3j m/s at point P having coordinates (1,2,4). Consider a plane through P which is normal to the vector b=-i+2k. What is the speed at which the fluid passes through the plane?

Should I do the dot product of the position vector P=[1,2,4] …
but what is the relevance of the position vector? hmm...maybe i dont need it?? ok just ignore the position vector part...

should i just find the dot product of v and a unit normal vector of the plane?

tiny-tim
yup! 