PHY024 · Fluids · Classes 9–11
Archimedes principle
Objective: To verify Archimedes' principle: the upthrust on an immersed body equals the weight of liquid it displaces.
🧰 Apparatus
- Spring balance
- Metal block on a thread
- Overflow (Eureka) can
- Small beaker
- Measuring cylinder
- Water
📐 Formulas and SI units
F_B = ρ_f g V (N)
apparent weight = W − F_B
💡 Theory
A body in a liquid is pushed up by a buoyant force F_B = ρ_liquid g V_displaced. When fully immersed it displaces its own volume, so it seems lighter by exactly the weight of liquid it pushes out. A body floats when this upthrust can equal its weight before it is fully under.
🪜 Procedure
- Weigh the block in air with the spring balance.
- Fill the overflow can until water runs out of the spout; let it stop.
- Lower the block fully into the water and collect the overflow in the beaker.
- Read the apparent weight.
- Weigh (or measure) the displaced water.
- Compare the loss of weight with the weight of displaced water.
📋 Observations Simulated values
✅ Result and conclusion
A 100 cm³ block in water displaces 100 g of water, so the upthrust is 0.98 N; an aluminium block (2.65 N in air) reads 1.67 N in water.
🔬 In a real lab
Drops cling to the spout and beaker, and the spring balance has ±0.05 N resolution, so measured loss and overflow differ by a few per cent.
⚠️ Safety
Wipe up spilt water at once; keep water away from electrical sockets.
❓ Quick check
1. The buoyant force equals the weight of…
2. A ship floats because…
3. In salt water the upthrust on the same block is…
🏋️ Practice
EasyVolume 200 cm³ in water. F_B?
Answer: 1.96 N
MediumWeight 5 N, apparent weight 3 N. F_B?
Answer: 2 N
ChallengeVolume of the object in the previous question?
Answer: ≈ 204 cm³