Question

A small source of sound vibrating at frequency $500Hz$ is rotated in a circle of radius $100/\pi cm$ at a constant angular speed of $5.0$ revolutions per second. The speed of sound in air is $330m/s$.
For an observer situated at a great distance on a straight line perpendicular to the plane of the circle , through its centre , the apparent frequency of the source will be

• A. greater than $500Hz$
• B. smaller than $500Hz$
• C. always remain $500Hz$
• D. greater for half the circle and smaller during the other half

Answer 1

The correct option is C always remain $500Hz$

Since the source is moving on a small circle in a plane perpendicular to the direction of the wave the observer located on the axis of the circle , there would be no change in the observed frequency which will be the same as the real frequency , i.e., $500Hz$