You blow a whistle of frequency 769 Hz, as you move towards a wall.

The frequency of beats you hear from the whistle and the reflected sound is 11 beats/s.

How fast are you moving towards the wall?


Give your answer in m/s and to two significant figures.

Only answer in numerical values, no units.


769 + 11 = 780hz. Which I believe is the new frequency

You notice that the observed frequency which is the frequency reflected from the wall is 780Hz. Now you have to take one step further by relating the change in frequency with the speed to travelling towards the wall. Recall that the doppler effect is the one which helps you to explain the phenomenon.

By f' =(c+u)f/c
f' is the apparent frequency
c is the speed of sound in air
f is the original frequency of sound produced by the whistle
u is the speed of travelling

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Going to face the Hong Kong Advanced-Level Examination which is believed to be the most difficult AL exams in the world this year. I hope good results can be achieved.