Tangential and normal components

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In mathematics, given a vector at a point on a curve, that vector can be decomposed uniquely as a sum of two vectors, one tangent to the curve, called the tangential component of the vector, and another one perpendicular to the curve, called the normal component of the vector. Similarly, a vector at a point on a surface can be broken down the same way.

Illustration of tangential and normal components of a vector to a surface.

More generally, given a submanifold N of a manifold M, and a vector in the tangent space to M at a point of N, it can be decomposed into the component tangent to N and the component normal to N.

Formal definition

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Surface

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More formally, let   be a surface, and   be a point on the surface. Let   be a vector at  . Then one can write uniquely   as a sum   where the first vector in the sum is the tangential component and the second one is the normal component. It follows immediately that these two vectors are perpendicular to each other.

To calculate the tangential and normal components, consider a unit normal to the surface, that is, a unit vector   perpendicular to   at  . Then,   and thus   where " " denotes the dot product. Another formula for the tangential component is  

where " " denotes the cross product.

These formulas do not depend on the particular unit normal   used (there exist two unit normals to any surface at a given point, pointing in opposite directions, so one of the unit normals is the negative of the other one).

Submanifold

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More generally, given a submanifold N of a manifold M and a point  , we get a short exact sequence involving the tangent spaces:   The quotient space   is a generalized space of normal vectors.

If M is a Riemannian manifold, the above sequence splits, and the tangent space of M at p decomposes as a direct sum of the component tangent to N and the component normal to N:   Thus every tangent vector   splits as  , where   and  .

Computations

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Suppose N is given by non-degenerate equations.

If N is given explicitly, via parametric equations (such as a parametric curve), then the derivative gives a spanning set for the tangent bundle (it is a basis if and only if the parametrization is an immersion).

If N is given implicitly (as in the above description of a surface, (or more generally as) a hypersurface) as a level set or intersection of level surfaces for  , then the gradients of   span the normal space.

In both cases, we can again compute using the dot product; the cross product is special to 3 dimensions however.

Applications

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References

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  • Rojansky, Vladimir (1979). Electromagnetic fields and waves. New York: Dover Publications. ISBN 0-486-63834-0.
  • Crowell, Benjamin (2003). Light and Matter.