Consider a particle which moves in a plane along a fixed curve, such that at a given instant it is at position s, measured from point o. The t axis is tangent to the curve at the point and is +ve in the direction of increasing s. +ve direction will be designated with the unit vector ut. A unique choice for the normal axis can be made by noting that geometrically the curve is constructed from a series of differential arc segments ds. Each segment ds is formed from the arc of an associated circle having a radius of curvature will be designated to as the curvature and center of curvature o". The normal axis n is perpendicular to the t axis w/ its +ve sense directed toward the center of. This +ve direction, which is always on the concave side of the curve, by the unit vector un.