Preparation time: 5 minutes
Supplies: string of length 1 or 2 meters, mobile phone, sock
Procedure:
You will need a mobile phone that can be programmed with user generated ring tones. Program a ring tone which consists of one note repeated again and again for a period of at least 20 seconds. Demonstrate to the class that the ring tone consists of just the one note. Now place the phone in the sock, tie it to the string, and swing the string rapidly around your head so that the phone moves in a large circle around you. As the phone moves towards the students, they will hear the pitch increased, and as it moves away, they will hear the pitch decreased, because of the Doppler Effect. Note that for the person swinging the phone, their phone neither approaches nor moves away from their ears, but circles around them, so there will be little or no discernable Doppler Effect.