The angle subtended by the vector $A=4 \hat{\mathbf{i}}+3 \hat{\mathbf{j}}+12 \hat{\mathbf{k}}$ with the $X$-axis is

$N$ number of balls of mass $m \mathrm{~kg}$ moving along positive direction of $X$ - axis, strike a wall per second and return elastically. The velocity of each ball is $u \mathrm{~m} / \mathrm{s}$. The force exerted on the wall by the balls in newton, is

Metal rings $P$ and $Q$ are lying in the same plane, where current I is increasing steadily. The induced current in metal rings is shown correctly in figure

A 10 m long wire of resistance $20 \Omega$ is connected in series with a battery of emf 3 V and a resistance of $10 \Omega$. The potential gradient along the wire in $\mathrm{V} / \mathrm{m}$ is