Velocity of fluid through a point on a plane

[racnna]

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 I am getting a negative velocity

 

[tiny-tim]

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?

 

[racnna]

hmm...maybe i don't 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!