Doppler effect.

The Doppler effect describes the change in frequency observed for a moving source and/or receiver. A common example of the Doppler effect involves a moving vehicle sounding a siren or horn as it approaches, passes, and recedes from an observer. The observed frequency (compared to the emitted frequency) is higher during the approach, is identical at the instant of passing by, and is lower during the recession. In this presentation, the Doppler Effect is demonstrated using a buzzer embedded in a Nerf ball. A difference in pitch can be heard when the ball is thrown to or from an individual. A second demonstration involves twirling a tuning fork from an attached string. An increase/decrease in frequency can be heard as the tuning fork moves toward/away from an observer.