An observer is riding on a bicycle and moving | vedclass

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(C) The velocity of the observer is $V_{o} = 18\,km/h = 18 	imes \frac{5}{18} = 5\,m/s$. The source is stationary,so $V_{s} = 0$.For the direct sound

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$20$

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(C) The velocity of the observer is $V_{o} = 18\,km/h = 18 	imes \frac{5}{18} = 5\,m/s$. The source is stationary,so $V_{s} = 0$.For the direct sound,the observer is moving away from the source. The apparent frequency is given by:$f_{	ext{direct}} = f_{0} \left( \frac{v - V_{o}}{v} ight) = 640 \left( \frac{320 - 5}{320} ight) = 640 \left( \frac{315}{320} ight) = 2 	imes 315 = 630\,Hz$.For the reflected sound,the hill acts as a stationary source of frequency $f_{0} = 640\,Hz$. The observer is moving towards the hill (towards the reflected source). The apparent frequency is given by:$f_{	ext{reflected}} = f_{0} \left( \frac{v + V_{o}}{v} ight) = 640 \left( \frac{320 + 5}{320} ight) = 640 \left( \frac{325}{320} ight) = 2 	imes 325 = 650\,Hz$.The beat frequency is the difference between the two frequencies:$f_{	ext{beat}} = f_{	ext{reflected}} - f_{	ext{direct}} = 650\,Hz - 630\,Hz = 20\,Hz$.

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