Doppler effect (source and detector moving together)

[Any Help]

Homework Statement

A source emits sound with a frequency of 860 Hz. It is moving at 20.0 m/s toward a stationary reflecting wall. If the speed of sound is 343 m/s, what frequency does an observer riding with the source hear?

• 
  A
  
  860 Hz
• 
  
  B
  
  913 Hz
• 
  
  C
  
  910 Hz
• 
  
  D
  
  765 Hz
• 
  E
  
  967 Hz

Homework Equations

WHERE FL and vL are the frequency and velocity of sound heard by the personL
fs and vs are the frequency and velocity of sound emmited by the source

The Attempt at a Solution

:[/B]
v=343 m/sec
vs=20 m/sec
vL=-vs=-20 m/sec since if it is moving away from the source
fs=860Hz
then using the aboove equation

FL=860Hz=Fs
but it is incorrect? Why?

 

[Doc Al]

You need to do this in two steps. Step one: What is the frequency received at the wall? (Then treat the reflected sound from the wall as the new source.)

 

[Any Help]

You need to do this in two steps. Step one: What is the frequency received at the wall? (Then treat the reflected sound from the wall as the new source.)

Ah then vs=vL since both are coming toward each other.
then fL=976 Hz which is correct :) thanks

 

[Any Help]

You need to do this in two steps. Step one: What is the frequency received at the wall? (Then treat the reflected sound from the wall as the new source.)

Suppose we don't have a wall, then how are we supposed to find that frequency received by the person?

 

[Doc Al]

Suppose we don't have a wall, then how are we supposed to find that frequency received by the person?

Not sure what you mean. What is he hearing? (There needs to be a source of sound.)