To introduce the unit vectors of the local coordinate system shown in Figure 1, an approach is to begin in Cartesian coordinates in an inertial framework and describe the local coordinates in terms of these Cartesian coordinates. In Figure 1, the arc length s is the distance the particle has traveled along its path in time t. The path r (t) with components x (t), y (t) in Cartesian coordinates is described using arc length s (t) as: r ( s ) = [ x ( s ) , y ( s ) ] .