# Homework Help: Surface normal directions

1. Sep 2, 2010

### physiker99

How do I find "surface normal direction" for a plane on a point with coordinates specified?

Last edited: Sep 2, 2010
2. Sep 2, 2010

### ponjavic

Do you have a plane equation or do you only have the coordinates of a point? Please be more specific.

3. Sep 2, 2010

### physiker99

i need to find normal directions for r^2=9 and x+y+z^2=1 at the point (2,-2,1)

4. Sep 2, 2010

### gabbagabbahey

First, neither of those surfaces are planes. The first is a spherical shell, and the second is a paraboloid.

Second, straight from wikipedia:

I'd be shocked if this wasn't in your notes or textbook, and I strongly suggest you review the section of your text that covers this.

Anyways, just like wikipedia implies, to find a surface normal at a point, define $F$, take the gradient (in the appropriate coordinate system) and plug in the point.

Last edited by a moderator: May 4, 2017