An open organ pipe P of length 0.85 m is kept at rest in still air. Another identical pipe Q is mounted on a trolley that can move along a straight track towards or away from a stationary observer O. Both pipes are excited in their fundamental modes at the same instant.
The observer hears 3 beats per second when the trolley is set in motion. Assume the speed of sound in air to be 340 m s⁻¹.
(i) Calculate the fundamental frequency of pipe P.
(ii) Determine the speed of the trolley when it is moving towards the observer.
(iii) What will be the beat frequency heard if the trolley moves with the same speed but in the opposite direction? Give numerical value.