曲率とその解釈

Curvature measures how sharply a curve bends at a given point, distinguishing gentle bends from sharper ones. To analyze these bends in space, consider a curve defined by a position vector r(t), where t represents time. The unit tangent vector is the normalized derivative of the position vector.

Curvature is the magnitude of the change in the unit tangent with respect to arc length. When tracking motion over time, it is calculated by dividing the time-derivative of the tangent vector by the speed.

The derivative of the unit tangent vector is always orthogonal to it and points toward the center of the curvature.

A familiar real-life example of a space curve is a helix, which can be found in a coiled spring or a spiral staircase.

The curvature of a helix depends on its radius, the distance from the central axis, and its pitch, which measures the vertical rise per radian.

As the pitch increases, the curvature decreases. When the pitch is zero, the helix becomes a circle, and curvature equals the reciprocal of the radius. If the radius is zero, the curvature vanishes, as expected for a straight line.