Doppler Ball

Physics, Astronomy:	Sound, Waves, Doppler Effect
Grade Range:	Elementary School, Middle School, High School
Format:	Stage

This demonstration uses a foam ball and a noisemaker to demonstrate the Doppler Effect. This demonstration easily connects to everyday life for most students, and they will enjoy knowing an answer to one of the most common phenomenon they experience!

Materials

• Doppler Ball
• A 9V battery

Safety Precautions

Please see the General Safety Precautions in the Demonstration Safety Page.

Demonstration

1. Show the Doppler Ball to the students. Turn it on briefly to let them hear the sound, and explain that they need to listen to that sound as you throw the ball back and forth with your volunteer.
2. Turn on the ball and start tossing it back and forth from across the room. If students are in bleachers or seats, toss it back and forth in front of them. If they are sitting on the ground, have your volunteer go behind the crowd and throw it over their heads.
3. Ask students to describe how it sounded:
   1. While it was coming towards them.
   2. While it was moving away from them.
4. Ask for ideas on why the pitch of the sound increased as it came towards them and decreased as it went away from them.

Why This Works

Short Explanation

Sound is a type of Wave. Sound waves are compression waves, which means they travel through matter, like air, by making pockets of high and low pressure. Sound waves travel pretty fast, but we can hear the difference in how fast an object is moving by listening to how the sound coming from it changes. When the ball is traveling towards you, the sound waves are slightly closer together, which increases the pitch you hear. The opposite is also true; when the ball travels away from you, the sound waves are slightly farther apart, which decreases the pitch. This change in pitch is known as the Doppler Effect, which states that the sound you hear from a source can have a different pitch if the source is getting closer or farther from you.

The Doppler Effect also applies to Astronomy, since it is an effect that happens with all waves! With light, we call it the Redshift/Blueshift Effect. If you are observing a distant star, it might appear red even if the star isn't red. This means that the star is moving away from us, so the light waves coming from it are slightly farther apart, making it appear more red! likewise, if the star is moving towards us, the light waves might appear slightly closer together, which would make it look more blue!

Full Explanation

Coming Soon

Tips and Tricks

• Often we hear this effect when we listen to a train whistle as the train passes by or when a police car with its siren blaring approaches and then passes and moves away. Another example is a race car that "vrooms" past. The effect happens whenever there is a sound emitting source moving in a radial direction from the observer.
• This demonstration is a part of the Astronomy Show.