PURPOSE: To illustrate terminal velocity quantitatively.

DESCRIPTION: Stacks of different numbers of coffee filters are dropped from various heights. The viscous force is proportional to the square of the velocity, so the terminal velocity is proportional to the square root of the number of filters in the stack. A stack of four filters will have a terminal velocity twice that of a single filter; the stack of four will therefore take the same time to fall two meters that a single filter takes to fall one meter, as is being demonstrated by Gwen in the photograph.

SUGGESTIONS: Practice.

See Question of the Week #64 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA 2C30.65) Papers in the Demonstration Reference File describe complexities with the terminal velocity of something falling through the atmosphere.

EQUIPMENT: Coffee filters, 2 meter stick.

SETUP TIME: None.