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 displacement of a plane progressive wave in a medium, travelling towards positive x-axis with velocity 4 m/s at t = 0 is given by $$y = 3\sin 2\pi \left( { - {x \over 3}} \right)$$. Then the expression for the displacement at a later time t = 4 sec will be