Which tense is used in the following sentence: 'They are playing football.'

If the kinetic energy of a particle of mass $m,$ performing uniform circular motion in a circle of radius $r,$ is $E,$ find the acceleration of the particle.

Consider the following radioactive decays

I. ${}_{92}\mathrm{U}{\stackrel{-\mathrm{\alpha }}{⟶}}_{90}\mathrm{Th}$ and

II. ${}_{90}\mathrm{Th}\stackrel{-\mathrm{\alpha }}{⟶}{}_{88}\mathrm{Ra}$

In which case group of parent and daughter elements remains unchanged.

Two particles $\mathrm{A},\mathrm{ }\mathrm{B}$ 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: