Lecture 7 Slides

```Doppler effect in sound
• An approaching source sounds higher
in pitch than the source.
• A source that is moving away sounds
lower in pitch than the source.

Doppler Shift From a Moving
Observer
Doppler Shift From a Moving
Source
v  vo
f '
f
v  vs
v  the velocity of the wave
Approaching-use top sign --- Moving away-use bottom sign
Three observers,A,B, and C are listening to moving source of sound. The
diagram below shows the location of the wavefronts of the moving source with
respect to the three observers. Which of the following is true?
A.
B.
C.
D.
E.
The wavefronts move faster at A than at B and C.
The wavefronts move faster at C than at A and B.
The frequency of the sound is highest at A.
The frequency of the sound is highest at B.
The frequency of the sound is highest at C.
A stationary observer hears a pitch from a
source that moves towards him, the
observed frequency will be ____________
the source frequency
1. Lower than
2. The same as
v  vo
f '
f
3. Higher than
v  vs
v  the velocity of the wave
Approaching-use top sign --- Moving away-use bottom sign
Moving Source
1.
2.
3.
An observer approaches a stationary
frequency source, the observed frequency
will be ____________ the source frequency
Lower than
The same as
Higher than
v  vo
f '
f
v  vs
v  the velocity of the wave
Approaching-use top sign --- Moving away-use bottom sign
Moving Observer
Higher
Frequency is _______________
when approaching each other.
Lower
Frequency is _______________
when moving apart.
v  vo
f '
f
v  vs
v  the velocity of the wave
Approaching-use top sign --- Moving away-use bottom sign
Dogma?
"Don't interfere with anything in the Constitution. That
must be maintained, for it is the only safeguard of our
liberties." Abraham Lincoln
@sqcomic Forgive me but that sounds not unlike a religious
dogma...
Thomas S. Monson, April 2012 General
Conference
The toy boats of childhood had no keel for stability, no
rudder to provide direction, and no source of power.
Inevitably, their destination was downstream-the path of
least resistance.
Unlike toy boats, we have been provided divine attributes
to guide our journey. We enter mortality not to float with
the moving currents of life but with the power to think, to
reason, and to achieve.
Thomas S. Monson, April 2012 General
Conference
Our Heavenly Father did not launch us on our eternal
voyage without providing the means whereby we could
receive from Him guidance to ensure our safe return. I
speak of prayer. I speak too of the whisperings from that
still, small voice; and I do not overlook the holy scriptures,
which contain the word of the Lord and the words of the
prophets-provided to us to help us successfully cross the
finish line.
The Bell Tower plays a G (392 Hz) as I drive
past at 20 m/s (about 40 mph). What do I
hear?
v  vo
f '
f
v  vs
v  343m / s (velocityof soundin air)
vo  20 m / s
vs  0 m / s
Approaching
Moving away
The Bell Tower plays a G (392 Hz) as I drive
past at 20 m/s (about 40 mph). What do I hear?
v  vo
f '
f
v  vs
Approaching
Moving away
v  vo
 343m / s  20m / s 
f 
392Hz  415Hz
v
343m / s


v  vo
 343m / s  20m / s 
f '
f 
392Hz  369Hz
v
343m / s


f '
415 Hz is a G sharp, 369 Hz is a G flat
A stationary observer hears a pitch from a source
that moves towards him at a half the speed of
sound. Then the source stops and the observer
moves at half the speed of sound towards the
source. In the second case the frequency the
observer hears is . . .
A.
B.
C.
Lower than before
The same
Higher than before
You stand on a platform at a train station and listen to a train
approaching the station at a constant velocity. While the train
approaches, but before it arrives, what do you hear?
A.
B.
C.
D.
E.
F.
The intensity
The intensity
The intensity
The intensity
The intensity
same.
The intensity
same.
and the frequency of the sound both increasing.
and the frequency of the sound both decreasing.
increasing and the frequency decreasing.
decreasing and the frequency increasing.
increasing and the frequency remaining the
decreasing and the frequency remainging the
Light is also Doppler shifted
Atomic spectra of galaxies moving away is
shifted to lower frequencies.
 Some are shifted more than others—they
must be moving away faster.
 The farther away, the faster they are
moving.
 The universe is expanding

f red  4.8 x1014 Hz
f green  5.5 x1014 Hz
for an observer raveling
t
t oward t helight
cv
cv
v  4.1x107 m / s  108 m ph  .14c
f green  f red
Practical use . . .
Bats emit ~60kHz bursts of sound and listen for
Doppler shifted reflections off victims.
With “Doppler radar” meteorologists can see how
storms move and police can measure your speed
Astronomers use Doppler shifts to determine how
stars and galaxies are moving.
The following figure shows the wavefronts generated by an
airplane flying past an observer A at a speed greater than that of
sound. After the airplane has passed, the observer reports
hearing
A.
B.
C.
D.
E.
1. a sonic boom only when the airplane breaks the sound barrier, then
nothing.
2. a succession of sonic booms.
3. a sonic boom, then silence.
4. first nothing, then a sonic boom, then the sound of engines.
5. no sonic boom because the airplane flew faster than sound all along.

afxTmE
Concorde pilot Peter Benn, from an interview with
BBC about being able to hear the sonic boom
while he was piloting
"You don't actually hear anything on board. All we
see is the pressure wave moving down the
aeroplane - it gives an indication on the
instruments. And that's what we see of Mach 1.
But we don't hear the sonic boom or anything
like that. That's rather like the wake of ship - it's
behind us.“
The four figures below represent sound waves emitted
by a source. Which picture represent a moving source
with a velocity less than the speed of sound?
On a particularly windy day the wind is
blowing at half the speed of sound. If I
am standing still and a cow is flying
towards me at the same velocity as the
wind, by what factor will I hear his mooing
shifted in frequency?
A : f’ = (1/2) f
B : f’ = (2/3) f
C : f’ = (3/2) f
D : f’ = 2 f
```