Doppler Football

Introduction

Tackling the Doppler effect is a snap with this fun demonstration. Score extra points with your students as you drive home the concepts of sound wavelength and frequency with an audible tone emitted from a foam football.

Concepts

• Sound waves
• Wavelength and frequency
• Doppler effect

Experiment Overview

The purpose of this activity is to demonstrate the Doppler effect with a buzzer inside of a foam football. The football will be tossed back and forth between participants who will observe the difference in perceived pitch of the buzzer as the ball travels toward or away from each person.

Materials

Safety Precautions

A 9-V battery has a low current and is considered safe. All participants should wear safety glasses. Follow all laboratory safety guidelines.

Prelab Preparation

1. Attach the battery clip with alligator ends to a 9-volt battery.
2. Attach one alligator clip from the battery to one of the exposed wires from the piezo buzzer. Note: Strip some of the insulation from the buzzer wires if needed for better contact.
3. Wrap any excess exposed wire tightly around the tip of the alligator clip for a secure connection.
4. Leave the second alligator clip unattached until ready to perform the demonstration (see Figure 1).

5. Wrap the connected wire around the battery so the alligator clip connection and the piezo buzzer are on opposite sides with the hole in the buzzer facing away from the battery (see Figure 2).
6. Secure the buzzer and wires connected to the battery with a rubber band.
7. Cut a piece of electrical tape a few centimeters longer than the opening in the foam football. Set aside until ready to perform the demonstration.

Procedure

1. Obtain the partially connected piezo buzzer and battery, foam football, and electrical tape.
2. Open the slit in the foam football and insert the battery with the buzzer facing one end of the football.
3. Place the free wires from the battery and the buzzer into the cavity in the football.
4. Connect the alligator clip to the end of the free buzzer wire, wrap any excess wire tightly around the clip, and quickly close the football. Note: Before the football is closed, the sound of the buzzer may be uncomfortable for some students. Instruct students to cover their ears with cupped hands until the sound is muffled by the football.
5. Seal the slit in the football with electrical tape.
6. Instruct students to note the pitch of the buzzer sound emitted from the stationary football.
7. Throw the football to a student volunteer who is at least 6 meters away and instruct the student to describe the pitch of the buzzer as the football approaches.
8. Instruct the volunteer to throw the football back and describe the pitch of the buzzer as the ball moves away.
9. Repeat with other volunteers. Note: For participation by the entire class, see the Tips section.

Student Worksheet PDF

12303_Student1.pdf

Teacher Tips

• Disconnect the battery as soon as the demonstration is complete to extend the life of the battery and the buzzer.
• Do not leave the electrical tape on the football longer than necessary as the adhesive may pull off some of the outer coating of the football. Peel the tape off slowly.
• Wrapping the wires around the battery and securing the buzzer and wires with a rubber band as in steps 5 and 6 of the Prelab Preparation will reduce the likelihood of the alligator clips touching and creating a short circuit during the demonstration.
• The instructor and students should wear safety glasses throughout the demonstration.
• Instruct students to throw the football with just enough force to reach the intended receiver. Do not allow students to spike the ball. Excessive force may dislodge a connection in the circuit.
• If space permits, line up half of the students on one side of the room and half the students on the other side. Throw the football to the first student in line across the room. The receiver then throws the football to the next student in line across the room and so on, in a back-and-forth pattern along each line. The two lines should be 6–8 meters apart—far enough to observe the Doppler effect and close enough so all students are able to throw the ball to the receiver. The demonstration may also be conducted in a gym or outdoors.
• If time permits, allow students to conduct investigations suggested by their answers to Question 6 on the Doppler Football Worksheet.
• Students may research and report on the many applications of the Doppler effect. See the Discussion section for suggestions.

Answers to Questions

1. Describe the difference in the pitch of the buzzer when it is stationary compared to when the football is coming toward an observer.
   

   When the buzzer was stationary it emitted a steady, high-pitched sound. As the football traveled toward an observer, the pitch sounded higher than the original pitch.

2. How does the pitch of the buzzer sound when the ball is moving away from an observer compared to the pitch when stationary?
   

   As the ball moved away from the observer, the pitch sounded lower than the pitch when the ball was stationary.

3. Does the pitch of the buzzer actually change when the ball is in motion? Explain the difference in pitch that was heard as the ball moved toward or away from the observer. Option: Draw a diagram to explain the observations.
   

   The actual pitch of the buzzer does not change. As the ball traveled toward an observer, the sound waves were received more frequently than they were actually produced by the buzzer. Therefore, the sound that this individual heard had a higher pitch than what was actually being emitted by the buzzer. The individual that observed the sound source moving away (the football passer) received the sound waves less frequently than they were actually produced by the buzzer. This observer heard a lower pitch compared to the actual pitch of the buzzer (see Figure 4).

4. If someone were able to run alongside the football at the same velocity as the ball, how would the pitch of the buzzer sound to that person?
   

   If an observer could travel at the same speed and direction as the football, the true frequency of the buzzer would be heard because there would be no relative motion between the sound source and the observer.

5. A person is standing at a roadway intersection. A police car is approaching with its siren continually sounding at a single frequency. As the car nears the intersection, it slows down, briefly stops right in front of the observer, and continues on. What changes in pitch will the observer hear as the police car approaches, stops, and then passes?
   

   The observer will hear a higher pitch than the actual siren emits as the police car approaches. As the car slows and then stops, the pitch will lower until it reaches the actual pitch of the siren when the car is directly in front of the observer. The pitch will sound lower than the actual pitch of the siren as the car accelerates away from the observer.

6. What variables other than the direction toward or away from the observer might be tested? List three questions that could be investigated with the Doppler football.
   

   Possible questions may include: Does the speed of the ball affect the perceived pitch of the buzzer?
   How is the pitch perceived if the ball is stationary and the observer moves toward or away from the ball?
   Does spinning the ball have an effect on the perceived pitch as it travels toward or away from an observer?
   How does the perceived pitch change if the ball is tossed straight up in the air?

Discussion

All sounds originate from a vibrating object. The rapid back-and-forth motion of a tuning fork is a familiar example of a vibration. When an object vibrates it causes the air molecules surrounding the object to move. The rapidly vibrating object compresses the air molecules together briefly, and when the object moves away from the air molecules, a less pressurized, low-density air pocket is created. This region of lower density and pressure is referred to as rarefaction (see Figure 3).

It is important to note that the air molecules do not necessarily travel away from the object. Instead, the vibrating air molecules transfer their energy to adjacent molecules, which then transfer the energy to other adjacent molecules in a chain reaction–type sequence. This sequence of events repeats as a second pulse emanates from the vibrating object. The regions of compressed and rarefied air “travel” away from the object. When the vibration is smooth and continuous, like that of a tuning fork, a continuous pattern of compressed and rarefied air travels away from the vibrating object in a pattern known as a sound wave. When the sound wave reaches an individual’s ear, the tiny hairs in the inner ear are set in motion and vibrate. These vibrations inside the ear are interpreted by the brain as sound.

When the difference in velocity between the source of sound and the observer is any value other than zero, the pitch of the sound the observer hears will change. This is known as the Doppler effect, named after Austrian physicist Christian Doppler (1803–1863). A common example of this is observed when a train approaches and then passes an observer while its horn is blaring. As the train approaches the observer, the pitch of the horn sounds higher because each sound wave is emitted from a location closer to the observer than the previous wave, resulting in a decrease in the effective wavelength and an increased frequency (higher pitch). After the train passes the observer, the pitch decreases since the movement of the source means that each sound wave or crest is coming from a location farther away from the observer, resulting in an increase in the effective wavelength (lower frequency). The Doppler effect is apparent whether the source of sound is moving toward the observer or the observer is moving toward the source of sound.

If a sound source of known frequency travels in a straight line toward one individual and away from a second individual, both individuals will hear a sound of a frequency different from the known frequency (see Figure 4). In this demonstration, the individual in front of the moving source (the football receiver) will receive the sound waves more frequently than they are actually produced by the buzzer. Therefore, the sound that this individual hears will have a higher pitch than what is actually being emitted by the buzzer. The individual that observes the sound source moving away (the football passer) will receive the sound waves less frequently than they are actually produced by the buzzer. This observer will hear a lower pitch compared to the actual pitch of the buzzer. If an observer could travel at the same speed and direction as the football, the true frequency of the buzzer would be heard because there would be no relative motion between the sound source and the observer.

The Doppler effect occurs in any type of wave, including electromagnetic waves, and has many applications. Police radar units are based on the Doppler effect by calculating the relative velocity of the returning microwave pulse that was emitted toward the vehicle. Weather forecasters use similar techniques to determine the speeds of weather formations. Astronomers use the blue–red shifting of light to determine the relative movement of astronomical objects with respect to the Earth. The Doppler effect is used in medical ultrasound tests to measure and visualize blood flow in the body.

References

Special thanks to Loren Lykins, Carlisle School, Price, TX, for bringing this activity to the attention of Flinn Scientific.

Bilash, B.; Maiullo, D. A Demo a Day™—A Year of Physics Demonstrations; Flinn Scientific: Batavia, IL, 2009; pp 222–223.

Ruiz, M. J.; Abee, J.; Doppler Football. The Physics Teacher. 2006, 44, 440–441.