法線ベクトルと従法線ベクトル

Imagine a roller coaster track spiraling upward like a helix.

The car's motion at every point on the curve, defined by the position vector, is described by three key directions: forward motion, sideways curving, and how the curve is twisting out of its own plane.

These three directions correspond to the tangent, normal, and binormal vectors, respectively. Together, they form the Frenet-Serret frame, which explains how a particle moves and the curve twists through space.

The unit tangent vector is derived from the first derivative of the position vector, r'(t), which represents the velocity of the car. It points along the direction of motion, showing the direction the roller coaster is heading at any moment.

The unit normal vector is derived from the derivative of the tangent vector; it is perpendicular to the tangent and points radially toward the central axis of the helix.

The binormal vector, found using the cross product of the tangent and normal vectors, is perpendicular to both. It shows how the track twists in space and helps set the roller coaster's orientation.

In a circular helix, like a spiral staircase or coiled spring, these vectors create a stable coordinate system.