The magnetic moment of an iron bar is $M$. It is now bent in such a way that it forms an arc section of a circle subtending an angle of $60^{\circ}$ at the centre. The magnetic moment of the arc section is
$\frac{3 M}{\pi}$
$\frac{4 M}{\pi}$
$\frac{M}{\pi}$
$\frac{2 M}{\pi}$
A uniform $\operatorname{rod} A B$ is suspended from a point $P$ ,at a variable distance $x$ ,from $A$ ,as shown in figure. To make the rod horizontal,a mass'$m$'is suspended from its end $A$ .Which set of variables will give a straight line when they are plotted?
$m, x^2$
$m, \frac1{x^2}$
$m, \frac{1}{x}$
$m, x$
A pipe $A$ is connected with other pipes $B$ and $C$ as shown in the figure.The areas of cross-section of $A, B$ and $C$ are respectively $\alpha, \frac{\alpha}{2}$ and $\frac{\alpha}{4}$ .If the velocities of flow of water through $A$ and $B$ are $10 \mathrm{~m} / \mathrm{sec}$ and $6 \mathrm{~m} / \mathrm{sec}$ ,respectively,then velocity of flow,$V_c$ along $C$ is

$21 \mathrm{~m} / \mathrm{sec}$
$12 \mathrm{~m} / \mathrm{sec}$
$28 \mathrm{~m} / \mathrm{sec}$
$18 \mathrm{~m} / \mathrm{sec}$
A particle of mass $m$ is suspended from a point O by a string of length $R$ .It is given a velocity $u=3 \sqrt{g R}$ at the bottom.The difference in tension at point $B$ and at the point $C$ is $m$ 
$6 mg$
$3 mg$
$4 m g$
$8 m g$
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