Starting from rest, your friend dives from a high cliff into a deep lake below, yelling in excitement at the thrill of free-fall on her way down. You watch her, as you stand on the lake shore, and at a certain instant your keen hearing recognizes that the usual frequency of her yell, which is 917 Hz, is shifted by 54.5 Hz. Is this shift an increase or a decrease in the frequency?
How long has your friend been in the air when she emits the yell whose frequency shift you hear? Take 344 m/s for the speed of sound in air and 9.80 m/s2 for the acceleration due to gravity.