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"·ur, where "R" is a constant (the radius of the circle) and ur is the unit vector pointing from the origin to the point mass. The direction of ur 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, uθ is perpendicular to ur 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: