Equilibrium Paradox - Scales And Pulley

Purpose

Counterintuitive demonstration involving pulley system.

Equipment

Two spring scales and pulley, assembled as photographed.

Suggestions

Let your students vote before performing the experiment.

Images

b3-14

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. 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?

References

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