Velocity Vector in Polar Coordinates

Theorem
Consider a particle $p$ moving in the plane.

Let the position of $p$ at time $t$ be given in polar coordinates as $\left\langle{r, \theta}\right\rangle$.

Then the velocity $\mathbf v$ of $p$ can be expressed as:


 * $\mathbf v = r \dfrac {\d \theta} {\d t} \mathbf u_\theta + \dfrac {\d r} {\d t} \mathbf u_r$

where:
 * $\mathbf u_r$ is the unit vector in the direction of the radial coordinate of $p$
 * $\mathbf u_\theta$ is the unit vector in the direction of the angular coordinate of $p$

Proof
Let the radius vector $\mathbf r$ from the origin to $p$ be expressed as:
 * $(1): \quad \mathbf r = r \mathbf u_r$


 * MotionInPolarPlane.png

From Derivatives of Unit Vectors in Polar Coordinates:

The velocity of $p$ is by definition the rate of change in its position: