Two motors in a factory are running at slightly different rates.One runs at 825 rpm and the other at 786 rpm.You hear the sound intensity increase and then decrease periodically due to wave interference.How long does it take between successive instances of the sound intensity increasing?
A)1.54 s
B)1.43 s
C)1.66 s
D)1.79 s

Question

Quizplus · Accepted Answer

The sound intensity will increase when the waves of the two motors are in phase (crest to crest and trough to trough) and will decrease when they are out of phase (crest to trough). 
The time between two instances of crest to crest or trough to trough is given by:
T = 1 / (f1 - f2)
Where f1 and f2 are the frequencies of the two motors.
In this case, f1 = 825/60 = 13.75 Hz and f2 = 786/60 = 13.1 Hz.
So, T = 1 / (13.75 - 13.1) = 1.54 s.
Therefore, the answer is A) 1.54 s.

Two Motors in a Factory Are Running at Slightly Different