# Advanced elasticity/Curvature

It helps to a know a bit about curvature when you start learning how to do buckling analysis. The following discussion goes through the derivation of some useful elementary results relating to curvature. You have already learned these in your introductory calculus course. However, you may have forgotten the details. So this is a refresher lesson.

## Tangent vector to a curve[edit | edit source]

Let be a vector valued function (curve) of the parameter . The unit tangent vector to the curve traced by the function is given by

Note that the "velocity" of a point on the curve is in the direction of the tangent. Therefore, the unit tangent vector and the unit velocity vector have the same value

A straight line has the equation

Taking the derivative with respect to we see that the tangent vector is constant, i.e., it does not change direction. Alternatively, we may say that the condition implies that the unit tangent vector does not change direction.

If the curve is not a straight line, then the quantity measures the tendency of the curve to change direction.

## Normal vector to a curve[edit | edit source]

The unit normal to the curve is defined as

## Curvature vector of a curve[edit | edit source]

The curvature vector is defined as the rate of change of the unit tangent vector with respect to the arc length. If measures the arc length, then the curvature vector is given by . Now, the "velocity" is given by

Then

Therefore the curvature vector has the same direction at the unit normal vector.

## Curvature[edit | edit source]

The **curvature** () of the curve is the length of the curvature vector. That means,

### Radius of curvature[edit | edit source]

To get a feel for the radius of curvature, consider the equation of a circle

where is the radius of the circle and are the unit basis vectors in the directions. Then the "velocity" is given by

and the unit tangent vector is

Differentiating with respect to ,

Therefore, the curvature of the circle is

This shows that the radius of the circle is the reciprocal of the curvature of the circle. The **radius of curvature** of any curve is defined in an analogous manner as the reciprocal of the curvature of the curve at a point.

### Curvature of plane curves[edit | edit source]

Let us now consider a curve in a plane . Let be the angle that the tangent vector to the curve makes with the positive -axis. Then we can write

where are the unit basis vectors in the directions.

Taking the derivative we have

Therefore

Using the chain rule

The curvature can then be expressed as

### Useful relation for the curvature of plane curves[edit | edit source]

If the plane curve is parameterized as

the curvature of curve can also be expressed as

If, in addition, , we have

Curvature of a plane curve |

** Proof:**
The tangent vector to the curve is given by

Therefore

Differentiating both sides with respect to ,

Now,

Plugging (2) back into (1) we get

The curvature is given by

Also

since

Plugging (3) and (5) into (4) gives

For the situation where we can parameterize the curve using to get . Then,

## Bibliography[edit | edit source]

- Varberg and Parcell,
**Calculus, 7th edition**, Prentice Hall, 1997. - Apostol, T. M.,
**Calculus Vol. I, 2nd edition**, Wiley, 1967.