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 sca le reads zero. When 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?
SUGGESTIONS: Let your students vote before performing the experiment.
EQUIPMENT: Two spring scales and pulley, assembled as photographed.
SETUP TIME: None.
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