Dulku – Physics 20 – Unit 4 – Topic L
Dulku – Physics 20 – Unit 4 – Topic L
Specific Outcome:
i. I can explain, qualitatively and quantitatively, the Doppler effect on a
stationary observer of a moving source.
The Doppler Effect
The Doppler Effect
If a stationary (not moving) point source
emits sound waves, they appear as follows:
The Doppler Effect
The Doppler Effect
If a sound source is not moving, the waves
that are emitted follow equation v = fλ
ex.If sound is emitted at 300 m/s with a
frequency of 75 Hz, the wavelength would
be 4.0 m using v = fλ.
If the sound source were moving away from
you or towards you, what would be different?
Dulku – Physics 20 – Unit 4 – Topic L
Dulku – Physics 20 – Unit 4 – Topic L
1
The Doppler Effect
If a point source moves while it emits sound
waves, they appear as follows:
The Doppler Effect
If the object is moving towards you:
its waves are compressed (shorter λ)
its frequency is increased (higher f)
If the object is moving away from you:
its waves are stretched (longer λ)
its frequency is decreased (lower f)
Dulku – Physics 20 – Unit 4 – Topic L
The Doppler Effect
Dulku – Physics 20 – Unit 4 – Topic L
The Doppler Effect
This is called the Doppler effect
It explains why an ambulance:
has a higher pitch as it approaches you
has a much lower pitch once it passes
If an object moves while it emits waves, its
frequency is Doppler shifted by its motion
Dulku – Physics 20 – Unit 4 – Topic L
Dulku – Physics 20 – Unit 4 – Topic L
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The Doppler Effect
The Doppler effect is described by:
where:
vw
(
ƒd =
)
vw ± v s
ƒs
ƒd = observed Doppler shifted
frequency for the waves (Hz)
vw = speed of the waves (m/s)
vs = speed of the source (m/s)
ƒs = original frequency (Hz)
Shifts: towards = higher f, away = lower f
Dulku – Physics 20 – Unit 4 – Topic L
The Doppler Effect
ex. Calculate the change in pitch of an ambulance. The
ambulance travels at 25.0 m/s with a siren that
whistles at 300 Hz. Assume the speed of sound in
the air is 330 m/s.
(
(
)
)
ƒd =
330 m/s
330 m/s – 25.0 m/s
ƒd =
330 m/s
300 Hz = 278.8…Hz
330 m/s + 25.0 m/s
300 Hz = 324.5…Hz
Dulku – Physics 20 – Unit 4 – Topic L
The Doppler Effect
A source that moves towards you (higher
frequency) will be solved with equation:
(
ƒd =
vw
vw - vs
)
ƒs
A source that moves away from you (lower
frequency) will be solved with equation:
(
ƒd =
vw
)
vw + v s
ƒs
Dulku – Physics 20 – Unit 4 – Topic L
The Doppler Effect
ex. Calculate the change in pitch of an ambulance. The
ambulance travels at 25.0 m/s with a siren that
whistles at 300 Hz. Assume the speed of sound in
the air is 330 m/s.
∆ƒ = |324.5…Hz – 278.8…Hz| = 45.7 Hz
Dulku – Physics 20 – Unit 4 – Topic L
3
The Doppler Effect
The speed of sound is
about 330 m/s (Mach 1)
The Doppler Effect
This also causes a cone of condensing water
vapour, shaped like the shockwave
If a source moves at
Mach 1, it breaks the
sound barrier and
produces a “sonic boom”
This happens because of
a round shockwave
Dulku – Physics 20 – Unit 4 – Topic L
The Doppler Effect
If a source moves faster
than the speed of
sound, it produces an
even more dangerous
shockwave, shaped like
a triangle
Once the sound barrier is exceeded, the
condensation cone goes away
Dulku – Physics 20 – Unit 4 – Topic L
The Doppler Effect
The shockwave is formed by the addition of
the individual wave fronts into a triangular
shape (ex. bullets, shells)
This may travel along
the ground, and
damage buildings
Dulku – Physics 20 – Unit 4 – Topic L
Dulku – Physics 20 – Unit 4 – Topic L
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The Doppler Effect
ex. shockwave formed by the Thrust SSC as it
broke the land speed record (and the sound
barrier)
Dulku – Physics 20 – Unit 4 – Topic L
The Doppler Effect
The Doppler
effect also
applies to how
stars are redshifted with the
expansion of the
universe
Dulku – Physics 20 – Unit 4 – Topic L
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