# Calculating surface normal vector

1. Aug 3, 2009

### andykol

Hello,
I am trying to calculate normal vector for surface F Nf (see attached picture) to calculate velocity component for U. I tried to get more info but got confused.
Please tell me how I can calculate Nf.

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2. Aug 3, 2009

### tiny-tim

Welcome to PF!

Hello andykol! Welcome to PF!
I'm not sure what you're asking

you don't need the normal vector

to calculate the normal component for U, all you need is the angle, θ, between Nf and U …

the normal component of U is simply Ucosθ.

3. Aug 3, 2009

### slider142

More information is necessary. In any coordinate system, a vector normal to another vector has a dot product of 0 with that vector. In 2 dimensions, this simplifies to the normal vector having the negative reciprocal slope of the vector it is normal to. you haven't given two-dimensional coordinates to the points, nor angles, so there is no way to calculate an absolute normal vector, but if the problem does not relate to any particular coordinate system (ie., there is no potential field; gravity, electromagnetism) then choose a convenient set of coordinates; ie., let that rectangle be horizontal with vertical sides; then the normal vector points along the positive x-axis. You still need the angle between F and N.

4. Aug 4, 2009

### andykol

But I need to calculate unit normal vector for surface F. I am looking for formula that calculates normal vector of surface with any given angle.

5. Aug 4, 2009

### tiny-tim

Sorry, I still don't understand

how is F defined?

do you have a particular example in mind?

6. Aug 4, 2009

### andykol

Hello tiny-tim,

I have attached detailed picture. I have to calculate unit vector for surface-E Ne. For given Cartesian coordinates, how I can calculate surface normal?
E.g. 2D Cartesian mesh- I got surface-E at an angle θ.

#### Attached Files:

• ###### Vector.JPG
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Last edited: Aug 4, 2009