PURPOSE: An elementary problem involving horizontal friction.

DESCRIPTION: The top end of a rod is supported by a string connected to a fixed point, with the bottom end floating freely on a small piece of styrofoam on the surface of the water. Which of the three drawings below best portrays the equilibrium position of the system: (a), (b), or (c).

SUGGESTIONS: Ask your class to vote on the result before doing the experiment.

See Question of the Week #25 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA unknown.)

EQUIPMENT: String and stick assembled on stand, styrofoam chip, and pan of water, as photographed.

SETUP TIME: 5 min.