Question

A train, blowing a whistle of frequency $f$, is standing in the railway yard. A person is running towards the engine. The frequency of the sound of the whistle as heard by the personn will be

• A. greater than $f$
• B. less than $f$
• C. equal to $f$
• D. greater or less than $f$ depending on the speed of the train.

Answer 1

Answer: (A)

In the given situation , an observer is moving towards a stationary source of sound , therefore by Doppler's effect the apparent frequency $f^{\prime }$ of whistle heard by person (observer) will be ,

$f^{\prime }=f\left(\frac{v+v_o}{v}\right)$ ,

where , $f=$ original frequency of sound ,

$v_o=$ velocity of observer ,

$v=$ velocity of sound in air ,

It is clear from above formula that , $f^{\prime }>f$ ,the frequency of whistle heard by the person is greater than the original frequency of whistle .