Why some objects float and others sink in liquids
Pressure is force acting per unit area, P = F/A, measured in pascals. A sharp knife cuts easily because its thin edge has a tiny area, while wide tyres on tractors spread weight to reduce pressure on soft ground. In a liquid, pressure increases with depth and acts equally in all directions, which is why dams are built thicker at the bottom.
When an object is placed in a liquid, the liquid pushes up on it with a force called buoyant force or upthrust. Archimedes’ principle states that this upthrust equals the weight of the liquid displaced by the object. An object floats if its density is less than that of the liquid, and sinks if it is greater. Relative density is the density of a substance divided by the density of water. For example, a block of relative density 0.6 floats with 60 percent of its volume underwater.
Why it matters
Ships made of steel float because their hollow shape displaces a large volume of water, and submarines dive by filling tanks with water. Lactometers check the purity of milk and hydrometers measure the density of liquids using Archimedes’ principle. Understanding pressure also explains why camels can walk on sand and why nails have pointed tips. These ideas return in fluid mechanics in Class 11.
Key terms
- Pressure
- The force acting perpendicularly on a surface divided by the area of that surface.
- Buoyant force
- The upward force a fluid exerts on an object partly or fully immersed in it.
- Archimedes’ principle
- An immersed object experiences an upthrust equal to the weight of the fluid it displaces.
- Relative density
- The density of a substance compared with the density of water, with no unit.