Define centre of curvature.

The term 'curvature' is used to describe how sharply a curve turns at any given point. It's a mathematical way to understand the bending of a curve, providing us with a numerical value that represents the curve's deviation from being straight. Imagine tracing a curved path with your finger; the more sharply you need to turn your finger, the higher the curvature at that point.

In mathematics, curvature (denoted by the Greek letter kappa, \( \kappa \)) is often defined using the concept of a derivative, which tells us how much a function is changing at a point. For a curve, the second derivative with respect to its arc length tremendously influences its curvature. To put it simply, a road with a mild bend has a low curvature, while a tight hairpin turn has high curvature.

The 'circle of curvature' might sound fancy, but it's essentially the circle that matches the curve's direction and bend at a specific point as closely as possible. It is to a curve what a best-fitting circle is to a set of points. At the point where the curve and the circle of curvature touch, they share the same tangent line—meaning they head in the same direction—and they also 'bend' at the same rate, which we describe using curvature.

When you try to best match a circular sticker's edge to a curved surface at a single point, you're finding a physical representation of the circle of curvature. It approximates the curve locally, making an exact fit at the point of contact. Think of a race car at the apex of a turn; the circle of curvature represents the ideal path it takes around the sharpest part of the bend.

We've talked about the circle that snuggly fits our curve, but what about the size of this circle? Enter the 'radius of curvature'. This measures the radius of the aforementioned circle of curvature. It's a straight shot from the center of the circle to its perimeter, and it quantitatively represents how