Which of the following alkali metals reacts directly with nitrogen to form a nitride?

If $y=\tan ^{-1}\left(\frac{\sqrt{1+a^2 x^2}-1}{a x}\right)$,then $\left(1+a^2 x^2\right) y^{\prime \prime}+2 a^2 x y^{\prime}$ is equal to

$A$ charged particle moves through a magnetic field in a direction perpendicular to it. Then the

$A$ hoop of radius $r$ and mass $m$ rotating with an angular velocity $\omega_0$ is placed on a rough horizontal surface. The initial velocity of the centre of the hoop is zero. What will be the velocity of the centre of the hoop when it ceases to slip?

$\frac{d}{dx}{\left( \sqrt{x} + \frac{1}{\sqrt{x}} \right)^2} = $

For the given figure,if the block remains in an equilibrium position,find the frictional force between the block and the wall in $N$.