# Homework Help: Unit normal vector

1. Sep 8, 2009

### t_n_p

1. The problem statement, all variables and given/known data

http://img171.imageshack.us/img171/5997/76283103.png [Broken]

3. The attempt at a solution

I think a good place to start is with the definition of regular.
My definition is: σ is regular if ∂σ/∂s and ∂σ/∂v are linearly independent.

Want to confirm I'm on the right track before going further!

Last edited by a moderator: May 4, 2017
2. Sep 9, 2009

### t_n_p

bump?
help anyone!?

3. Sep 9, 2009

### MathematicalPhysicist

regularity means that:
$$d\sigma =/=0$$ for any s and v which is the same as what you wrote, i.e equivalent statements.
And you know how to find the normal to sigma, just multiply by scalar product N with the tangent of sigma, which is the derivative, and then divide by its norm.

4. Sep 9, 2009

### t_n_p

ok, so after showing that sigma is regular, I want to find the unit vector normal (as you said above).

Found my equation in the text, just wondering if the notation is correct
http://img3.imageshack.us/img3/8527/11604435.png [Broken]

Last edited by a moderator: May 4, 2017
5. Sep 11, 2009

### t_n_p

to the top again

6. Sep 12, 2009

### t_n_p

Well, I'm assuming the definition posted 2 posts above is correct, so then I proceed to evaluate ∂σ/∂s and ∂σ/∂v..

∂σ/∂s = ∂γ/∂s + r[(dn/ds)(cos v) + (db/ds)(sinv)]

and

∂σ/∂v = r[-n(s)sin v + b(s)cosv]

But now I'm wondering, how do I cross product these two if they are not vectors!!

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