Two tuning forks A and B emit sound waves of nearly equal frequencies
PHYSICS · Multi-part · AP · Physics 2: Algebra-based · Waves Sound and Physical Optics · Waves Sound and Physical Optics
Problem
Two tuning forks A and B emit sound waves of nearly equal frequencies. Fork A has a frequency of 400 Hz and is kept at rest. Fork B initially at rest produces 2 beats per second with fork A.
Fork B is then fixed on a motor-driven cart that moves directly away from a stationary observer with a uniform speed of 8.7 m s⁻¹ along the line joining the forks and the observer. After the cart attains this speed, the observer hears 8 beats per second, and the pitch of the sound from fork B is lower than that from fork A.
Take the speed of sound in air as 340 m s⁻¹.
(i) Determine the actual frequency of fork B.
(ii) Show by calculation that the information given about the direction of motion is consistent with the change in beat frequency.
(iii) If the cart were to move towards the observer with the same speed, what beat frequency would be heard? Calculate the value.