Radius Vector

The Vector r from the Origin to the current position. It is also called the Position Vector. The derivative of ${\bf r}$ satisfies

$${\bf r}\cdot {d{\bf r}\over dt} = {1\over 3}{d\over dt} ({\b... ...{\bf r}) = {1\over 2} {d\over dt} (r^2) = r {dr\over dt} = rv,$$

where $v$ is the magnitude of the Velocity (i.e., the Speed).