GOCE
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The Doppler Effect
Background Information:
The Doppler Effect was named after Christian Doppler,
who first came up with the idea in 1842.
He thought that sound waves would have a higher frequency if the source was moving toward the
observer and a lower frequency if the source was moving away from the observer. The change in
pitch observed as train passes is an example of this phenomenon. Think of sound waves as pulses
emitted at regular intervals. Imagine that each time you take a step, you emit a pulse. Each pulse
in front of you would be a step closer than if you were standing still and each pulse behind you
would be a step further apart. In other words, the frequency of the pulses in front of you is
higher than normal and the frequency of the pulses behind you is lower than normal.
The red-shift observed from distant galaxies was the first
evidence that the Universe is expanding, supporting the Big Bang
Theory. Police radars do not directly measure the time that it
takes for a pulse to bounce back, but in fact the frequency shift
caused by the Doppler Effect.
To accurately locate the position of GOCE, satellite to
satellite tracking is used. This involves measuring the amount
of Doppler shift in signals sent to GOCE from several Global
Positioning satellites. Measurement of the way in which the
received frequency from a satellite transmitter changes as
the satellite passes by is a means of determining the
distance between the satellite and the receiver, and the
time of closest approach. As a satellite approaches, the
frequency appears raised relative to the actual transmission
frequency. As it goes away, the frequency appears to be
lowered. At the time of closest approach, the transmitted
and received frequencies are the same. You can even track
the flight path of GOCE on a world map.
Learning Objectives:
Pupils are aware that waves are shifted to higher frequencies when a source of sound
approaches and lower frequencies when a source of sound moves away. They will appreciate
that this can be used to monitor the flight path of satellites.
Outcomes:
Pupils print out three sound profiles- one for the stationary object, approaching object and
receding object. They label them in terms of frequency shift.
Curriculum Links:
Edexcel GCSE in Physics (2109)
P1 b 11.12: Compare the properties of longitudinal and transverse waves, giving examples of
each type, including sound waves, ultrasound, seismic waves and electromagnetic waves
P1 b 11.14: Explain the terms: amplitude, frequency, wavelength, speed of a wave
The Twenty First Century Science suite GCSE Physics A (J635)
SE16b: Distant galaxies are moving away from us. This means that the Universe is getting
bigger (expanding). The more distant a galaxy is, the faster it is moving away. This suggests
that the Universe might have begun in one place with a huge explosion (the ‘big bang’) about
13700 million years ago.
AQA Physics 2009 (4451)
11.7 If a wave source is moving relative to an observer there will be a change in the observed
wavelength and frequency.
There is a red-shift in light observed from most distant galaxies. The further away galaxies
are the bigger the red-shift.
How the observed red-shift provides evidence that the universe is expanding and supports
the ‘big bang’.
Suggested activities
Part I: Qualitative Doppler Effect
Materials:
Buzzer (constant pitch) and battery
Tennis ball
Plastic rulers
String
Clothes Pegs
Directions:
Attach a piece of string to the buzzer and battery. Pierce a small hole in the tennis ball and
push the string through with a screwdriver. Cut a slot on the other side so that pliers can be
used to pull the string through. Tie a big knot to hold it in place. Use the string to swing the
tennis ball with the buzzer inside it. Observe the changing pitch. Why does the pitch
change? Relate pitch to frequency. Use rulers of different lengths and ping them to get
different notes. It can be seen that the frequency rises with pitch. Revise the formula v=fλ.
Role Play To Explain The Doppler Effect
Ask a volunteer to represent the sound wave front. Tie a long piece of string around their
middle. Ask another student to represent the sound source. They hold the string. The class
should clap at a constant slow rate. The first pupil should take one step forwards on each
clap. They are moving at a constant velocity – the speed of sound. Every second clap, the
second student should place a clothes peg on the string. The marks are being made at a
fixed frequency. Stop after about 40 claps and hold the string to up to the board. Draw a
wave which has peaks matching the positions of the pegs.
Repeat the experiment again, but this time the pupil representing the sound source should
move in the direction of the sound wave but at half the speed of the wave i.e. one step every
two claps. They should continue to mark the string on every second clap. So they are moving
forward and placing a clothes peg on the string on alternate claps.
Again hold the string up to the board and draw a wave which has peaks matching the
positions of the pegs. The peaks should be closer together than previously. The frequency is
greater when the sound source is moving in the same direction as the wavefronts.
Discuss what happens at the speed of sound i.e. when the pupil representing the sound
source follows the other pupil at the same speed. Discuss what happens when the pupil
representing the sound source moves in the opposite direction to the pupil representing the
wavefronts.
Part II: Quantitative Doppler
Sound analysis software, such as the free Audacity package, can be used to demonstrate the
Doppler Effect. Details are available from:
www.mutr.co.uk/images/lam_sep_222_doppler_effect_unit.pdf
Alternatively, a similar experiment can be conducted in the school playground with a colleague
driving past at a known speed, sounding their horn. The sound profile can be recorded a
computer and analysed with Audacity, but this time a sonogram (a frequency/time plot) can be
used to observe the Doppler Effect. Alternatively there are plenty of audio recordings of cars
passing on the web. Some sites are listed at the end.
Extension:
The speed of the moving object can be found using the equation:
where is the velocity of waves in the medium,
is the velocity of the source relative to the
medium and is the velocity of the receiver relative to the medium.
References/Resources:
A video demonstrating the Doppler Effect:
http://web.ics.purdue.edu/~mjcarlso/ST/ST027_Doppler_Effect.mp4
A demonstration of the Doppler shift:
http://www.wfu.edu/physics/demolabs/demos/avimov/byalpha/cdvideos.html
Applets explaining the Doppler Effect
http://www.colorado.edu/physics/2000/bec/lascool3.html
Another Doppler Effect Applet
http://lectureonline.cl.msu.edu/~mmp/applist/doppler/d.htm
Doppler Effect with a car horn:
http://www.kettering.edu/~drussell/Demos/doppler/carhorn.wav
A video clip of a teacher demonstrating the Doppler Effect:
http://www.easy-kids-science-experiments.com/doppler-effect-science-experiment.html
An explanation of the Doppler Effect and a demonstration:
http://www.planet-scicast.com/experiment.cfm?cit_id=2697
Sonic booms:
http://www.kettering.edu/~drussell/Demos/doppler/doppler.html
The GOCE flight path is tracked by GPS and can be followed on this site:
http://www.esa.int/esaEO/SEMZ8TJTYRF_index_0.html