M-S 6: Influence of normal force on friction

PIRA: Unknown

Equipment: meter stick, two stand rods about 6 inches long, one as is, the other covered with rubber tubing to make a high friction surface. The meter stick is in rm 4 and the 2 stand rods Set 9, Cabinet 3, Shelf 2.

Procedure: The lecturer may perform the experiment or he may select a student for the purpose to reassure the students that there is no trick involved.

The "demonstrator" extends his two index fingers horizontally in front of him separated by perhaps half a meter. The meter stick is laid flat upon the extended fingers with the center anywhere between the two fingers. The question is: if the demonstrator now moves the fingers toward each other always keeping the meter stick horizontal, where on the scale will the two fingers come together? The experimental answer: always at the center of gravity, which, if the stick is uniform, is the 50 cm mark.

To emphasize the fact that the success of the experiment does not depend upon the contacts with the fingers being similar, replace one finger by the smooth rod and the other finger by the rubber-covered rod, and repeat the experiment.

The better student should be able to figure out this puzzle.