A train, standing in a station yard, blows a whistle of frequency… - Vidyadip

MCQ

A train, standing in a station yard, blows a whistle of frequency $400 \,\,Hz$ in still air. The wind starts blowing in the direction from the yard to the station with a speed of $10\,\,m/s.$ Given that the speed of sound in still air is $340\,\,m/s.$ Mark the INCORRECT statement :

A
The frequency of sound as heard by an observer standing on the platform is $400\,\,Hz.$
B
The speed of sound for the observer standing on the platform is $350\,\,m/s.$
✓
The frequency of sound as heard by the observer standing on the platform will increase.
D
The wavelength of sound as received by the observer standing on the platform will increase.

✓

Answer

Correct option: C.

The frequency of sound as heard by the observer standing on the platform will increase.

c
Frequency will change when there is relative motion between source and an observer therefore frequency will remain same.

$\mathrm{v}=\mathrm{n} \lambda$

Due to velocity of wind speed of sound increases $\mathrm{v}_{\mathrm{S}}=\mathrm{v}_{\mathrm{sound}}+\mathrm{v}_{\mathrm{o}}$

$\mathrm{v}_{\mathrm{s}}=[340+10]=350 \mathrm{m} / \mathrm{s}$

$\mathrm{v}=\mathrm{n}$

$350=400(\lambda)$

$\lambda_{\mathrm{AP}}=\frac{350}{400}>\lambda_{\text {original }}$

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