Fast Fact:
Police radar guns use the Doppler effect to determine whether cars are breaking the speed limit. The bigger the Doppler effect with the radar waves, the faster the car is going.
Objective:
Doppler shifting sounds will be demonstrated.
In the Film:
The vroom of cars going by is a familiar sound at the racetrack. The students will hear this sound during Super Speedway. As the cars flash by, the sound of the motors appears to change from a high pitch to a lower pitch, even though the cars are travelling at a constant speed.
Background:
When objects such as motors vibrate the air around us, we detect these airwaves as sounds. The pitch of the sounds we hear depends on the frequency with which the airwaves hit our ears. If the object is moving it can change the frequency of the waves. This effect was first analyzed by the Austrian physicist, J.C. Doppler (1803-1853).
Materials:
A cord, a buzzer or other noisemaker, a small bag or sock. Alternatively, a whistle attached to a plastic or rubber hose may be used.
To Do:
Choose one student to demonstrate. Place the buzzer in the sock and securely close the end of the socks with the cord. Leave several feet of cord free. Have the student swing the buzzer around on the end of the cord. Ask the other students to note when the pitch lowers. Is it when the object moves toward them, or away from them? A longer cord will make it easier for the students to hear the effect. (If some students have difficulty detecting the effect, instruct them to listen for a wawa sound.) Ask the demonstrator to increase the speed of rotation. What happens? Have the students describe how the changing sound of the buzzer is like the changing pitch of a car as it zooms by.
Whats Going On?
When the source of the vibration (the buzzer) is stationary, the sound waves around the source are uniform. When the source moves, the centers of the waves shift. The waves in front of the moving object are packed closer together and we hear a higher pitch, whereas the waves behind the object are further apart. At the racetrack the speed of a race car is great enough to create this thrilling effect.
Taking It Further:
1. Have the students make sounds at different pitches by vibrating objects (rulers on the edge of their desks) at various speeds. Ask them to note how the musical pitch of the sound is related to the frequency of the objects vibration.
2. Have the students research how the colors of stars shift from red to blue in astronomy. Ask them to show how the color change is similar to the pitch of the Doppler effect.