When sound is produced in an aeroplane with a velocity of 200 m/s horizontally its echo is heard after $$10\sqrt 5 $$ seconds. If velocity of sound in air is 300 ms^$$-$$1 the elevation of aircraft is

Two turning forks of frequencies n₁ and n₂ produces n beats per second. If n₂ and n are known, n₁ may be given by

A car moving with a velocity of 36 km/hr crosses a siren of frequency 500 Hz. The apparent frequency of siren after passing it will be

The equation of a stationary wave along a stretched string is given by $y=5 \sin \frac{\pi x}{3} \cos 40 \pi t$. Here $x$ and $y$ are in cm and $t$ in second. The separation between two adjacent nodes is