PURPOSE: To illustrate forces in a potentially paradoxical way.

DESCRIPTION: The mass on the hanger (center photograph) pulls down on the spring scale with a force W equal to its weight (150 gram force units on the spring scales shown in the photograph). Q: If two identical mass hangers are connected over the two pulleys, with an extended spring and an identical spring scale between the pulleys, what will that spring scale read: 0, W, or 2W? Suggest that it might be zero because the spring "uses up" the force in becoming extended. Alternatively, suggest that it might read 2W because each of the two masses on the ends is pulling with a force W. The photograph at the right shows the result.

SUGGESTIONS: Let your students guess before letting one read the result.

See Question of the Week #72 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA 1J30.20)

EQUIPMENT: Spring scale, pulley, and spring gizmo as photographed.

SETUP TIME: None.