Why do ships not sink?
A body immersed in a liquid or gas feels an upward push – the Archimedes (buoyant) force. Archimedes’ principle: it equals the weight of the displaced liquid: F_A = ρ_liquid × g × V_submerged. The ancient Greek scientist Archimedes lived in Syracuse in the 3rd century BC (about 287–212 BC; the textbook says the 2nd century, but it is the 3rd). The “Eureka” story of the crown is a famous legend. A ship is made of steel yet floats because it is hollow, so the hull displaces a large volume of water; the ship floats when the weight of displaced water equals the ship’s weight. A mark on the hull shows the deepest permitted loading (the waterline). Balloons rise by the same force: a balloon filled with helium or hot air has an average density smaller than the surrounding air. A person floats more easily in salt water because it is denser (the Dead Sea holds so much salt that people hardly sink).
Two experiments: 1) make a plasticine ball and a boat from the same plasticine, place them in a bowl of water and see which floats, then load the boat with coins to see how much it carries. 2) Put a boiled egg into plain water (it sinks), then into a container with 6 tablespoons of salt dissolved in 250 ml of water (it floats). Safety: do not drink or taste salt water; wipe spills (slipping); do not use raw egg and wash hands after handling eggs.