For a scalar field

find the normal unit vector to a surface s = 0 at the point (1,1,0)

I know i coudl work out grad for s(x,y,z), but not entirely sure how to go beyond this to get a normal unit vector :(

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- Jun 12th 2011, 11:01 AMimagemaniaUnit Normal Vector
For a scalar field

find the normal unit vector to a surface s = 0 at the point (1,1,0)

I know i coudl work out grad for s(x,y,z), but not entirely sure how to go beyond this to get a normal unit vector :( - Jun 12th 2011, 03:44 PMmr fantastic
- Jun 13th 2011, 02:09 AMimagemania
Well i know how to do it for parametrics. My attempt for this would follow something i did at a level:

So i assume the normal is one over these. And can sub in the coordinate points, thus obtaning:

Is this correct? - Jun 13th 2011, 02:54 AMPlato
- Jun 13th 2011, 03:51 AMimagemania
Yes,

sub in values, do unit vector:

hence:

(or could use -ve)

Im guessing this is the tangent unit vector?

Looking at a past question i think i can use the dot product here to ge the normal i.e.

Which after expanding i get a similar answer as above :\ (though 2 instead of a half). The reason i chose this method is because this is what we were taught to use - Jun 13th 2011, 03:53 AMAlso sprach Zarathustra
- Jun 13th 2011, 04:40 AMimagemania
I assume your partial derivatives should go x, y, z not x,x ,x as that wouldn't make sense.

Isn't that what i did?

which is where the root 9 came from.

But that itself cannot be the normal unit vector surely. Is that not the tangent unit vector and hence this has to be used to obtain the unit normal vector that is at right angles to this tangent (gradient) - Jun 13th 2011, 04:51 AMPlato
- Jun 13th 2011, 05:25 AMimagemania
I see this is a special case in teh notes of a level surface:

A surface defined explicitly, e.g. z = f (x, y), can be considered as a level surface of the scalar field

U(x, y, z) = z − f (x, y) + c, c = const.

The reason i thoguht not was i was comparing it to an example that wasn't a level surface, though i did not see this.

Thanks :) - Jun 13th 2011, 07:04 AMHallsofIvy
Yes, it was a level surface. You were asked to find the normal unit vector to a surface s = 0 which is clearly a level surface (the "level" being 0). To find a unit normal vector to a surface f(x,y,z)= constant, at a given point, evaluate at the given point and divide by its length.