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. (Click here for demonstration video.) If a net weight is placed on either side, that side will go down. (Click here for demonstration video.)

SUGGESTIONS: Ask your students what they think will happen when the system is released in the configuration photographed at the left above. Draw attention to the similarity between the Roberval balance and the simple pan balance, as seen in the two figures below.

See Question of the Week #1 for information on using this demonstration to enhance class involvement.

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.