TOPIC:

ID Code: B3-15
Purpose: Illustrate analysis of forces in a pulley system
Description: In the pulley system photographed, the weight hanging from the free pulley is W, and the pulleys are approximately massless. The rope will be pulled at its free end, and passes over the free pulley, under the pulley attached to the weight, and back over the fixed pulley to support the free pulley. With what force F must you pull on the free end of the rope to just barely lift weight W off the ground: W, W/2, W/3, or "other?"

Let your students guess before having one of them try to lift weight W by pulling on the end of the rope. Note that this is a "gag" demonstration! The reasons why the system stays set up as photographed are (1) the rope is pinned to the "free" pulley, and (2) the rope loop is stretched tightly between the upper and lower pulleys, so that the friction prevents the weight of the "free" pulley from falling. A video of the "action" is available below.

This result can be determined in about twenty seconds as follows: Pulling on the free end with a force F causes a tension F throughout the rope. The result is a force 2F downward and F upward on the "free" pulley, causing it to move downward.
Availability: Available