When an object is placed in water, it pushes some water out of the way. The water pushes back with an upward force called the buoyant force, and by Archimedes' principle this force equals the weight of the water displaced.
A solid block of steel displaces only a little water, whose weight is far less than the steel's own weight, so it sinks. A ship is built as a large hollow hull. As it settles into the sea it pushes aside a huge volume of water, and once the weight of that displaced water equals the ship's total weight, the upward buoyant force balances gravity and the ship floats.
This is why a ship rides lower when loaded with cargo, since it must displace more water to support the extra weight. The Plimsoll line marks the safe limit.
In SPM you should apply Archimedes' principle to explain floating and the role of displaced water.
Common misconceptions
- Heavy objects always sink and light ones always float -> What matters is whether the buoyant force from displaced water equals the object's weight, not weight alone.
- A ship floats because steel floats -> Steel sinks; the hollow shape displaces enough water to provide the buoyant force.
- The buoyant force depends on how deep the water is -> It depends on the weight of water displaced, not on the total depth of the sea.
The physics behind it
A ship floats because of upthrust, the upward force a fluid exerts on anything placed in it. Archimedes' principle states that the upthrust equals the weight of fluid displaced by the object.
For a fluid of density ρ, the upthrust is F = ρVg, where V is the volume of fluid pushed aside and g is the gravitational field strength. An object floats in equilibrium when the upthrust balances its weight, so it sinks just deep enough to displace its own weight of water.
Steel is denser than water, but a ship is not a solid block of steel; its hull is hollow and mostly air, so its average density is less than water's. Suppose a ship weighs 2 × 10⁶ N. To float, it must displace water weighing 2 × 10⁶ N. The volume needed is V = W/(ρg) = 2 × 10⁶ N ÷ (1000 kg m⁻³ × 9.81 m s⁻²) ≈ 204 m³.
The wide hull sinks until that much water is pushed aside, and then it floats in equilibrium.
See it in daily life
You can test the idea in a kitchen sink. A lump of modelling clay dropped in sinks straight away, because it displaces only its own small volume of water and the upthrust is too little.
Flatten the same clay into a bowl shape and it floats, because the hollow now pushes aside much more water, so the upthrust rises to match its weight. A ship is a giant version of that clay bowl.
The same principle explains the Plimsoll line painted on a ship's side: as cargo is loaded the ship rides lower, displacing more water, until the waterline reaches a safe limit. A ship also floats a little lower in fresh water than in denser sea water, because fresh water gives less upthrust for the same volume.
When you float on your back in a swimming pool, your body is doing exactly what the ship does, sinking until it displaces its own weight of water.
How this comes up in SPM
In Paper 2 this is examined under command words such as State, Explain and Calculate, in the pressure chapter alongside Pascal's principle and pressure in liquids. A standard question asks you to state Archimedes' principle, then explain why a steel ship floats although steel is denser than water, expecting you to argue from average density and displaced water.
Calculations often use upthrust F = ρVg, giving the density of the fluid and a volume and asking for the upthrust, or the volume needed for a given weight to float, so keep density in kg m⁻³, volume in m³ and force in N. You may be asked to explain the principle of flotation, that a floating object displaces its own weight of fluid, or to account for a hydrometer, a submarine or the Plimsoll line.
Be ready to compare floating in sea water and fresh water using density, and to state that upthrust arises because the pressure is greater at the deeper bottom of the object than at its top.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)