Practical: Conditions for floating
If a body’s average density is greater than the liquid’s, it sinks; if equal, it hovers inside the liquid; if smaller, it floats at the surface. In terms of forces: when fully immersed, if the buoyant force is less than the weight the body sinks, if equal it hovers, and if greater it rises to the surface and floats; for a floating body the buoyant force equals exactly its weight. The submerged fraction of a floating body equals the density ratio ρ_body ÷ ρ_liquid. For example wood of density 600 kg/m³ is 60% submerged in water. Ice (900 kg/m³) is about 90% submerged, which is why most of an iceberg lies under water. An empty sealed test tube floats high; with sand added its average density rises, it sinks deeper and with enough sand sinks completely. Swimming is good for health, but only swim under adult supervision and in permitted places.
Test-tube experiment: pour water into a tall clear container (measuring cylinder or tall glass). Put in a sealed plastic test tube or small bottle empty, then a quarter full of sand and then full of sand, each time measuring the depth to which it sinks with a ruler. Safety: use a plastic tube or small plastic bottle, not a glass test tube; keep containers away from table edges; wipe spills; keep sand out of eyes.