PURPOSE: To illustrate forces and the way they react to a pulley.

DESCRIPTION: The mass on the hanger in the center pulls down on the spring scale with a force W equal to its weight. Q: If two identical mass hangers are connected over the two pulleys, with a 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.

SUGGESTIONS: We can also do this demo with blackboard mechanics with or without a datastudio setup.

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

SETUP TIME: 50 Min.

KEYWORDS: force around a pulley