Physics 105, Fall Term, 2009
1) The Doppler Effect is
an observed shift in frequency caused by the motion of the observer or
2) The actual frequency of the sound is
not shifted, just the frequency that the observer notices seems different.
The Doppler Effect is a topic that everyone has experienced in their lives.
Whether it is the change in pitch of an ambulance siren as it passes you on the
road or the changing pitch of racecars going past the cameras when you turn on
ESPN looking for sports and get stuck with NASCAR. The Doppler Effect really
happens with all waves, but we are only able to recognize it in daily life with
sound waves. Go to:
http://www.colorado.edu/physics/2000/applets/doppler.html to see a physical
representation of the sound waves relationship to an observer when a source of
sound is moving.
Increase the speed of the police car by moving the scroll bar all the way to the
right. To an observer that the cop car is traveling toward, how do the
wavelengths compare to when the car is not moving?
The wavelengths are closer together
The wavelengths are farther apart
The wavelengths are the same
To an observer that the cop car is traveling away from, how do the wavelengths
compare to when the car is not moving?
How would the pitch of a siren sound to you (compared to the actual pitch of the
siren) when a police car is moving
The pitch would sound higher
The pitch would sound lower
The pitch would sound the same
How would the pitch of a siren sound to you (compared to the actual pitch of
the siren) when you are sitting in a police
A police radar gun is an application of the Doppler Effect. It works by sending
out an electromagnetic wave of a specific frequency at a car and measuring the
frequency of the wave as it reflects off the car. It then takes the difference
between the emitted frequency wave and returned frequency wave to calculate your
speed. Why must the the police measure your speed while you are headed directly
toward the gun?
If the radar gun is pointed in a direction parallel to the direction you are traveling, it will always measure the same frequency with which it was emitted. The reason it can measure a frequency shift when it is measuring a head on motion of a car is because the motion of that car will reflect the waves closer to each other if it is heading toward the gun.
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Was there anything that you didn't understand in the reading assignment?
What was confusing to you?