A man standing 25 m away from a wall produces a sound and receives the reflected sound. (a) Calculate the time after which he receives the reflected sound if the speed of sound in air is 350 ms^-1. (b) Will the man be able to hear a distinct echo? Explain the answer.

A RADAR sends a signal to an aeroplane at a distance 300 km away, with a speed of 3 x 10⁸ ms^-1. After how much time is the signal received back after reflecting from the aeroplane?

A ship on the surface of water sends a signal and receives it back from the submarine inside water after 4 s. Calculate the distance of the submarine from the ship. (The speed of sound in water is 1450 ms^-1).

A pendulum has a frequency of 5 vibrations per second. An observer starts the pendulum and fires a gun simultaneously. He hears an echo from the cliff after 8 vibrations of the pendulum. If the velocity of sound in air is 340 ms^-1, find the distance between the cliff and the observer.