A source of sound moves towards a stationary observer with a speed one third of sound. If the frequency of the sound from the source is 100 Hz, the apparent frequency of the sound heard by the observer is:
Correct answer: D. 150 Hz
- A. 60 Hz
- B. 100 Hz
- C. 200 Hz
- D. 150 Hz
Explanation
Doppler effect formula is=F0 = (v+v0/v +vs) fsf0= observed frequencyv=velocity of sound in air (330m/s)vs velocity of the sourcev0= velocity of the observerIf we rearrange the equation according to our case which is when a source is moving towards a stationary observer, the equation we get is:F0 = fs(v/v-vs)vs= one third of velocity of sound = 330/3 = 110v0= 0 because the observer is stationaryThe sign is negative since the source is moving toward the observer and the sign would be positive in case the source is moving away from the observer.Solution:F0 = 100(330/330-110)F0= 100(330/220) = 100x 1.5 =150HzHz
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