Two particles A, B are moving on two concentric circles of radii $R_{1}$ and $R_{2}$ with equal angular speed $\omega$. At $t = 0$, their positions and direction of motion are shown in the figure: The relative velocity $\overrightarrow{v_{A}} - \overrightarrow{v_{B}} at t = \frac{\pi}{2\omega}$ is given by: [add image]