Reaction To Buoyant Force


To demonstrate the reaction force using a liquid.


Pan balance, two 250ml beakers with water, 200g and 100g masses.


Ask your students to predict the result before doing the experiment.




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.


Meiners, Physics Demonstration Experiments, Section 8-1.8, pp. 139-140. ◙David A. Ward, Finding the Buoyant Force, TPT 32, 114-115 (1994). ◙Ronald Ebert, Letter: Does the Buoyant Force Depend on the Weight or Density, TPT 32 262-263 (1994). ◙Van E. Neie, Letter: Beware of Greeks Bearing Pan Balances, TPT 33, 6 (1995). ◙David A. Ward, Letter: Ward Responds, TPT 33, 6-7 (1995).