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Buoyancy & Archimedes' Principle Worksheet
By the end you will: predict whether things float or sink from their density, find the buoyant force, and use Archimedes' principle and relative density.
A. Explore with the simulation
Drop each object in Water. Does it float or sink? If it floats, what percentage is under the surface? Compare with its density.
Object Density (kg/m³) Floats or sinks? % under water Cork 240 Pine wood 500 Ice 917 PVC plastic 1380 Iron 7870 Drop the ice in Spirit, Water and Sea water. What happens each time? Why?
Choose Iron and Spring balance. Lower it fully into each liquid and fill in the table.
Liquid Weight in air (N) Balance reading (N) Loss of weight (N) Water Honey Drop the Iron into Mercury. What happens, and why?
B. Calculate (then check in the simulation)
A 1 litre aluminium block (2,700 kg/m³) hangs fully in water. Find its weight, the buoyant force and the spring balance reading. (g = 9.8 m/s²)
A stone weighs 24 N in air and 16 N fully under water. Find the buoyant force, its volume and its relative density.
A block of wood floats with 60% of its volume under water. What is its density?
C. Think it through
An iron nail sinks, but a ship made of iron floats. Explain.
Answer key (for teachers and parents)
- Cork floats, 24% · Wood floats, 50% · Ice floats, 92% · PVC sinks · Iron sinks. Anything less dense than water (1000) floats, and % under water = its density ÷ 1000.
- Spirit (789): ice sinks, because it's denser. Water: floats 92% under. Sea water (1025): floats a little higher, about 89% under. Denser liquids push up harder.
- Water: 77.1 N in air, about 67.3 N, loss 9.8 N · Honey: 77.1 N, about 63.2 N, loss 13.9 N. The loss equals the weight of 1 litre of the liquid pushed aside.
- It floats with about 58% under, because mercury (13,534 kg/m³) is denser than iron (7,870 kg/m³).
- Weight = 2.7 × 9.8 ≈ 26.5 N. Buoyant force = weight of 1 litre of water = 1 × 9.8 = 9.8 N. Reading = 26.5 − 9.8 ≈ 16.7 N.
- Buoyant force = 24 − 16 = 8 N. Volume = water pushed aside = 8 ÷ (1000 × 9.8) ≈ 0.00082 m³ (0.82 litres). Relative density = 24 ÷ 8 = 3.
- Fraction under = density ÷ water's density, so density = 0.6 × 1000 = 600 kg/m³.
- The ship is hollow, so its average density (iron plus the air inside) is less than water's. It pushes aside a huge volume of water whose weight equals the ship's weight before it sinks too deep.
Free to print and share for teaching. Made with the interactive simulation at jugnix.com/simulations/buoyancy.