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

In summary, the conversation discusses the concept of the Doppler Effect and how it applies to sound waves reflecting off a moving object. The question asks for the frequency of the reflected sound as heard by an observer on the moving object. The first solution presented uses the equation for the Doppler Effect and determines the frequency to be 2533hz. The second solution takes into account the speed of sound in air and calculates the frequency to be 2839.65hz, which is the correct answer. The conversation also touches on the assumption that the frequency of the source does not change on reflection.
  • #1
ReMa
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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:
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  • #2


The frequency of source does not change on reflection. Could you explain the thought behind your assumption in the first part?
 
  • #3


aim1732 said:
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


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.
 

1. What is the Doppler Effect?

The Doppler Effect is the apparent change in frequency or wavelength of a wave due to the relative motion between the source of the wave and the observer. This effect is commonly observed in sound waves, light waves, and other types of waves.

2. How does the Doppler Effect apply to a moving observer riding on a moving source with reflection off a wall?

In this scenario, the observer and the source of the wave are both in motion, and the wave is reflected off a wall. The observer will perceive a change in the frequency of the wave due to the combined effects of their own motion and the motion of the source. This change in frequency is known as the Doppler shift.

3. How does the speed of the observer and the source affect the Doppler shift?

The speed of the observer and the source will affect the amount of the Doppler shift. If both the observer and the source are moving towards each other, the frequency of the wave will appear higher to the observer. If they are moving away from each other, the frequency will appear lower.

4. How does the reflection off the wall impact the Doppler shift?

The reflection off the wall does not directly impact the Doppler shift. However, it does affect the overall distance between the observer and the source, which can also impact the perceived frequency of the wave.

5. What are some real-world applications of the Doppler Effect with a moving observer and source?

The Doppler Effect with a moving observer and source is used in various technologies, such as radar and sonar, to determine the speed and direction of moving objects. It is also used in medical imaging, such as ultrasound, to measure blood flow and heart rate.

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