During circular motion, the body moves on a curve that can be described in the polar coordinate system as a fixed distance R from the center of the orbit taken as the origin, oriented at an angle θ (t) from some reference direction. See Figure 4. The displacement vector r is the radial vector from the origin to the particle location: r ( t ) = R u ^ ^ R ( t ) , where u ^ ^ R ( t ) is the unit vector parallel to the radius vector at time t and pointing away from the origin. It is convenient to introduce the unit vector orthogonal to u ^ ^ R ( t ) as well, namely u ^ ^ θ θ ( t ) . It is customary to orient u ^ ^ θ θ ( t ) to point in the direction of travel along the orbit.