Let r (t) be a vector that describes the position of a point mass as a function of time. Since we are assuming circular motion, let r (t) = R · u r, where R is a constant (the radius of the circle) and u r is the unit vector pointing from the origin to the point mass. The direction of u r is described by θ, the angle between the x-axis and the unit vector, measured counterclockwise from the x-axis. The other unit vector for polar coordinates, u θ is perpendicular to u r and points in the direction of increasing θ. These polar unit vectors can be expressed in terms of Cartesian unit vectors in the x and y directions, denoted i ^ ^ and j ^ ^ respectively: u r = cos ⁡ ⁡ θ θ i ^ ^ + sin ⁡ ⁡ θ θ j ^ ^ and u θ θ = − − sin ⁡ ⁡ θ θ i ^ ^ + cos ⁡ ⁡ θ θ j ^ ^ .