PURPOSE: To determine the direction of the frictional force in a possibly ambiguous situation.
DESCRIPTION: Place weights in the box and hanging from the pulley such that the system is in static equilibrium. Add mass to the box until it begins to slip down the incline; the frictional force must be in the upward direction. Hang additional weight on the string over the pulley until the box begins to slide up the incline; the frictional force must be in the downward direction.
SUGGESTIONS: Relate this demonstration to the general friction problem in which the direction of the frictional force must be determined.
REFERENCES: (PIRA unavailable.)
EQUIPMENT: Inclined plane with pulley, friction box with string, 200 g mass, 8 50 g masses.
SETUP TIME: None.
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