PURPOSE: To show that the sum of forces exerted on the mass by the scales is constant.

DESCRIPTION: The weight is set on the scale platform, and the upper spring scale hooked to the weight. As an upward force is applied, the sum of the readings of the two spring scales remains constant, equal to the initial weight on the platform. In this case the sum of the forces is slightly less than 10N as more of the lifting is transferred to the upper scale.

SUGGESTIONS: See Question of the Week #202 for suggestions on using this demonstration to enhance class involvement.

REFERENCES: (PIRA unknown.)

EQUIPMENT: Spring scale, platform spring scale, 1 kg mass, as photographed.

SETUP TIME: None.