PURPOSE: Counterintuitive demonstration involving pulley system.

DESCRIPTION: A frame containing the pulley and the lower scale hangs from the upper scale as photographed. The initial weight of the lower scale, pulley, and frame together is about 5 Newtons, as read on the upper scale; initially the lower scale reads zero. With the frame detached, you can exert a force on the scale by pulling on the free end of the rope passing over the pulley. With the frame hanging onto the upper pulley, if you pull gently on the free end of the rope until the lower scale reads 10 Newtons, what will the upper scale read: 5N, 10N, or 15 N? Click your mouse on the picture at the right to see the result.

SUGGESTIONS: Let your students vote before performing the experiment.

See Question of the Week #26 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA 1M20.15) See Demonstration Reference File for original sources of this demonstration.

EQUIPMENT: Two spring scales and pulley, assembled as photographed.

SETUP TIME: None.