Archimedes’ Principle
Materials: ★★☆ Available in most school laboratories or specialist stores
Difficulty: ★☆☆ Can be easily done by most teenagers
Safety: ★☆☆ Minimal safety procedures required
Categories: Density and Buoyancy
Alternative titles: Buoyant Force and Displacement
Summary
This demonstration shows that the buoyant force on an object immersed in a fluid equals the weight of the fluid displaced. By comparing aluminum and brass masses of equal weight, the difference in apparent weight loss reveals the effect of density on buoyant force.
Procedure
- Fill a large beaker (about 2 liters) with water.
- Attach cotton loops to two equal-mass objects (e.g., 1 kg of aluminum and 1 kg of brass).
- Suspend each mass from a spring balance and record its weight in air.
- Slowly immerse the aluminum mass in the beaker of water and observe the decrease in the reading on the spring balance.
- Repeat with the brass mass and compare the apparent weight reduction.
- Note that the less dense aluminum displaces more water and thus experiences a greater buoyant force.
Links
Conceptual Physics: Demo of Archimedes' principle - Marshall Ellenstein:
Archimedes Principle demonstration - KClassScienceChannel:
📄 Archimedes' Principle - Harvard Natural Sciences Lecture Demonstrations: https://sciencedemonstrations.fas.harvard.edu/presentations/archimedes-principle
Variations
- Try objects of different materials but equal volume to compare differences in buoyant force.
- Repeat the experiment in liquids of different densities (e.g., saltwater, oil) to show how buoyancy depends on the fluid.
- Use irregular objects (like a stone or crown replica) to illustrate Archimedes’ original problem.
Safety Precautions
- Handle heavy masses carefully to avoid dropping them.
- Ensure the beaker is on a stable surface to prevent spills.
- Dry equipment after use to prevent slipping hazards.
Questions to Consider
- Why does the aluminum mass experience a greater buoyant force than the brass mass? (Because aluminum has lower density, so it displaces more water when submerged.)
- How does Archimedes’ principle explain why ships made of steel can float? (Although steel is dense, the overall structure displaces enough water to balance its weight.)
- How did Archimedes use this principle to test the king’s crown? (By comparing the crown’s displacement in water to that of pure gold of the same weight.)