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