PURPOSE: To show the direction of the frictional force when a rolling object is accelerated.

DESCRIPTION: One of the three cylindrically symmetric rollers is positioned on the base as shown in the photograph. The base has a rubber top surface and rolls almost without friction on small bearings on the horizontal surface below. The cloth web is held horizontally and yanked, causing the roller to move in the direction of the applied force. The frictional reaction force on the cart is indicated by the direction the cart will move when the roller is yanked off. For a solid disc (moment of inertia less than mr^2) the reaction force is in the direction opposite to the pull, and the base will move backward. For a spool rolling on its smaller radius r (moment of inertia greater than mr^2) the reaction force is in the direction of the applied force. For a thin ring (moment of inertia equal to mr^2) there is no reaction force on the base and it will remain motionless.

SUGGESTIONS: Practice; operating this can be tricky.

REFERENCES: (PIRA unavailable.) See Demonstration Reference File for lots of information on this demonstration, including a paper written for The Physics Teacher magazine describing this apparatus.

EQUIPMENT: Three symmetric rollers with various moments of inertia, base with rubber top and frictionless lower surface.


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