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Doppler Effect: Moving Observer riding on moving source! With reflection off wall!

  • Thread starter ReMa
  • Start date
  • #1
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Homework Statement



A bus is moving at 37.00m/s towards a wall. The sound from the bus has an original wavelength of 0.1500m. The sound from the bus reflects off the wall. What frequency sound does an observer on the moving bus hear from the reflection??

Homework Equations



Moving Observer: fo = fs (1 + vo/v)
v = LaTeX Code: \\lambda f

The Attempt at a Solution



Is this doppler effect??

vo = 37.00m/s

Since v = LaTeX Code: \\lambda f

vi = LaTeX Code: \\lambda fi
f = v / LaTeX Code: \\lambda
= (343m/s)(0.1500) = 2286.667hz

Subbing into equation:

fo = (2286.667hz)(1 + 37m/s / 343m/s) = 2533hz



Ok, I ALSO tried another method...

Vs = 37 m/s

Therefore the speed of the wavefront is: vs + v
where v = 343m/s (speed of sound in air)

fs = 343m/s / 0.15m = 2286.667hz

Frequency observed is thus:

(v+vs/v-vs)fs = (343+37 / 343-37) (2286.667) = 2839.65hz

Both of these answers are choices in the multiple choice part, so this is becoming a frustrating question for me.

Help appreciated! Thanks!
 
Last edited:

Answers and Replies

  • #2
430
2


The frequency of source does not change on reflection. Could you explain the thought behind your assumption in the first part?
 
  • #3
10
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The frequency of source does not change on reflection. Could you explain the thought behind your assumption in the first part?
I was confused about that too, but am now thinking I misread some of my notes... which would explain A LOT of why that didn't make sense to me.

Could you check my two possible solutions and let me know if either is correct?
 
  • #4
430
2


Well the second one is right.
I was thinking if frequency and velocity of propogation of sound do not change in reflection wavelength shouldn't change too.
 

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