Home > Physics > Classical Mechanics: Lecture 23 Notes

Classical Mechanics: Lecture 23 Notes


Today is a 50 minute lecture on the doppler effect.


Speed of Sound = 340 m/s

f’ > f if the sound approaches

f’ < freceeding

A transmitter with the speed of 1m/s.  It approaches with f’=1.003f.  It recedes with f’ = .997f

Rotate the transmitter in a circle.  A circle with radius r, the listener in the plane of
the circle. 

This change in frequency as the radiation source moves is called the doppler effect. 

f’max/f = v transmitter

2 pi R/T = v transmitter
We can find radius, period and speed of rotation.

He does a demonstration with a wind organ, a tube he whips around his head. 

Electromagnetic (EM) waves travel with the speed of light.  All that matters in special relativity
is relative motion. 

For v << c
The radial component is the component in your direction. 
f’ = f (1 + vcos0/c)

With sound, something is vibrating.  With EM radiation, charges are vibrating.
 T = 1/f
Wavelength = ct = c/f
c = wavelength x frequency

Red light is low energy, blue light is high energy.  The visible spectrum varies from
400 to 700 nm. 

Blue shift is when the source is approaching you and the new frequency is higher than the old.
Red shift is the opposite. 

w’/w = 1.000032 means that a red shift, then one can find velocity as well. 

Goes goes through a long detour into astrophysics and stars.  It’s just an application of what we’ve learned here. 


c = wf
t = 1/f
Red is low, blue is high. 


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