# Homework Help: Moving observer and sound

1. Dec 8, 2013

### Tanero

1. The problem statement, all variables and given/known data
A source of sound emits waves at a frequency f 450 Hz. An observer is located at a distance d 150 m from the source. If the observer is moving away from the source at a velocity vobs 40 m/s, how does the number of wavefronts change with time? dN/dt ? (in Hz)

2. Relevant equations
λ=u/f
N=d/λ
u - speed of sound

3. The attempt at a solution
with moving observer my frequency changes. f'=(u/u+vobs)*fsource
λ'=u/(u/u+vobs)*fsource)=(u+vobs)/f

in time Δt, ΔN=(d+vobsΔt)/λ' - d/λ'

then I find a limit for Δt>0 lim ΔN/Δt= dN/dt

Am I on the right way?

2. Dec 8, 2013

### Simon Bridge

1 wavefront arrives per cycle.
The number of wavefronts that arrive per unit time is the number of cycles per second - what quantity is measured in cycles per second?

3. Dec 8, 2013

### Tanero

Just frequency? No need of all that ΔN/Δt calcs?

Then, if observer is receding f' = u/[(u+vobs)*fsource]
Is it? dN/dt = f' . Is that what you mean?

4. Dec 8, 2013

### Simon Bridge

The number of wavefronts observed per second is the frequency of the sound observed :)

It's an odd way of asking for it though - and what does the distance to the source have to do with it?
It sounds to me like there may be some information missing from the problem statement.
Like if the observer was originally stationary and at t=0 suddenly starts moving ... then the frequency would change with time.