State Archimedes' principle. Describe an activity with a spring balance to show that a body loses weight in water, and give two uses of the principle.

1. 1 · State the principle. When a body is fully or partly immersed in a fluid, it feels an upward force equal to the weight of the fluid it displaces.
2. 2 · Set up. Hang a stone from a spring balance by a thread and note its weight in air.
3. 3 · Draw the apparatus. [Diagram: spring balance on a stand; stone on a thread fully in a beaker of water; labels: spring balance, thread, stone, water, beaker]
4. 4 · Observe. As the stone goes into the water, the reading falls. Once it is fully under, the reading stops changing.
5. 5 · Explain. Water exerts an upward buoyant force. Loss of weight = buoyant force = weight of the water displaced.
6. 6 · Two uses.
   • Designing ships and submarines
   • Hydrometers and lactometers (purity of milk)
7. 7 · Close. So a body in a fluid is pushed up by the weight of the fluid it displaces.

Scoring tip: Draw the stone fully under water and clear of the beaker's bottom. Write 'weight of the fluid displaced' in full.