PURPOSE: To demonstrate a paradox in equilibrium of forces and torques.
DESCRIPTION: This unlikely-looking contraption is in neutral equilibrium when equal weights are placed onto the two outer arms, as shown, so it will remain at rest in any position. If a net weight is placed on either side, that side will go down.
SUGGESTIONS: Ask your students what they think will happen when the system is released in the configuration photographed. Draw attention to the similarity between the Roberval balance and the simple pan balance.
REFERENCES: (PIRA 1J40.50) See several interesting articles in the Demonstration Reference File.
EQUIPMENT: Roberval balance with two 200gram weights and one 50 gram weight.
SETUP TIME: None.
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