PURPOSE: To demonstrate equilibrium of forces.
DESCRIPTION: Three forces are set up in static equilibrium. The lower spring scale is modified so that it measures its own weight plus the weight hanging on the hook below. Upper spring scales are calibrated to read zero when the connecting arms are in place and the lower scale is removed. A large protractor can be used to measure angles directly. Using a point source of light, the lines can be projected onto a screen or blackboard. Masking tape vectors, with lengths proportional to the magnitudes of the forces, can be positioned along the shadows, then translated to show that the vector sum of forces is zero.
REFERENCES: (PIRA 1J30.50) See also Demonstration B2-12.
EQUIPMENT: Three spring scales with connecting arms mounted on board on rolling frame, 1.5 kg mass, protractor, point source of light.
SETUP TIME: 5 min.
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