Two particles execute S.H.M. of same amplitude and frequency along the same straight line path. They pass each other when going in opposite directions, each time their displacement is half the amplitude. The phase difference between them is $\left(\sin 30^{\circ}=0 \cdot 5\right)$
The pitch of whistle of an engine appears to drop by $30 \%$ of the original value when it passes a stationary observer. If speed of sound in air is $350 \mathrm{~m} / \mathrm{s}$, then the speed of engine in $\mathrm{m} / \mathrm{s}$ is
A monoatomic ideal gas, initially at temperature ' $\mathrm{T}_1$ '' is enclosed in a cylinder fitted with a frictionless piston. The gas is allowed to expand adiabatically to a temperature ' $\mathrm{T}_2$ ' by releasing the piston suddenly. $L_1$ and $L_2$ are the lengths of the gas columns before and after the expansion respectively. Then $\frac{T_1}{T_2}$ is