Friction Direction On Inclined Plane

Purpose

To determine the direction of the frictional force in a possibly ambiguous situation.

Equipment

Inclined plane with pulley, friction box with string, 200 g mass, 8 50 g masses.

Suggestions

Relate this demonstration to the general friction problem in which the direction of the frictional force must be determined.

Images

c6-04

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.

References

Julius Sumner Miller, The Paradox of the Stillson Wrench, AJP 22, 236 (1954). ◙Thomas D. Phillips, Finding the External Force, AJP 22, 583-585 (1954). ◙F. H. Northover, Problem of the Stillson Wrench, AJP 23, 276-278 (1955). ◙Peter Froehle, Tricks of the Trade: Centripetal Force Provided by Friction, TPT 30, 299 (1992).