An engine is moving on a circular path of radius $200 \mathrm{~m}$ with speed of $15 \mathrm{~m} /…
An engine is moving on a circular path of radius $200 \mathrm{~m}$ with speed of $15 \mathrm{~m} / \mathrm{s}$. What will be the frequency heard by an observer who is at rest at the centre of the circular path, when engine blows the whistle with frequency $250 \mathrm{~Hz}$?
Less than $250 \mathrm{~Hz}$
Greater than $250 \mathrm{~Hz}$
$250 \mathrm{~Hz}$
Zero
Solution
Since the engine is moving on a circular path and the observer is at the center of the circle, the distance between the engine and the observer remains constant. Therefore, there is no relative radial motion between the source (engine) and the observer. The Doppler effect occurs only when there is relative motion along the line joining the source and the observer.
Since there is no change in the distance between the source and the observer, the frequency heard by the observer remains the same as the source frequency-in this case, 250 Hz .