# physics

A boy is blowing a whistle of frequency 536 Hz and walking toward a wall with a speed of 1.64 m/s. What frequency of the reflected sound will the boy hear if the speed of sound is 343 m/s?

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asked by Jon
1. The frequency is shifted upward twice, each time by a relative amount 1.64/343 = 0.0048. That amounts to 0.0048*536 = 5 Hz each time. The frequency he hears is therefore 10 Hz higher, or 546 Hz.

The sound frequency is shifted by 5 Hz as received at the wall, and the boy hears it shifted another 5 Hz because he is walking towrd the wall the produces the echo.

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posted by drwls
2. Thank you drwls. You have a little math error though the answer is 541. I understand everything you did. Can you PLEASE be the person to check my work PLEASE?

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posted by Jon
3. Yes, you are right. I doubled the frequency shift twice. I am glad you caught my error.

I did not use the fancier equation for Doppler shift that has both Vs and Vd in it, although it is more accurate, because the difference is small when the speed of the source or detector is much smaller than the speed of sound.

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posted by drwls

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