PURPOSE: To demonstrate the reaction force using a liquid.
DESCRIPTION: A beaker of water is balanced by two brass weights. Q: If you stick your finger into the water about up to the first knuckle, will (a) the water beaker side go down, (b) the weight side go down, or (c) the system remain in equilibrium? A: The water side will go down. The water exerts a buoyant force on your finger, so your finger exerts a reaction to the buoyant force on the water, thereby causing the water side to go down.
SUGGESTIONS: Ask your students to predict the result before doing the experiment.
REFERENCES: (PIRA 2B40.12.) See Demonstration Reference File.
EQUIPMENT: Pan balance, two 250ml beakers with water, 200g and 100g masses.
SETUP TIME: None.
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