| English | Archimedes' principle |
|---|---|
| Bahasa Melayu | Prinsip Archimedes |
| 中文 | 阿基米德原理 |
Definition
An object wholly or partly immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid it displaces.
What you need to know
Archimedes' principle states that an object fully or partially immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid it displaces. This upward force, called the buoyant force, explains why some objects appear lighter in water and why others float.
The buoyant force is calculated using Fb = ρVg, where Fb is the buoyant force in newtons (N), ρ is the density of the fluid in kilograms per cubic metre (kg m⁻³), V is the volume of fluid displaced in cubic metres (m³), and g is the gravitational field strength in metres per second squared (m s⁻²).
For a floating object, the law of flotation applies: the buoyant force acting on the object is exactly equal to the weight of the object itself, so the resultant force is zero and the object remains in equilibrium at a fixed depth. An object sinks when its weight is greater than the maximum possible buoyant force, which occurs when the object is fully submerged and still weighs more than the fluid it displaces.
Worked example
A solid block of volume 0.002 m³ is fully submerged in water of density ρ = 1000 kg m⁻³. Calculate the buoyant force acting on the block, taking g = 9.81 m s⁻².
Since the block is fully submerged, the volume of water displaced equals the volume of the block, so V = 0.002 m³.
Using Fb = ρVg, the buoyant force is Fb = 1000 kg m⁻³ × 0.002 m³ × 9.81 m s⁻² = 19.62 N.
If the actual weight of the block is greater than 19.62 N, the block will sink, because the buoyant force is not enough to support it. If the weight of the block were instead exactly 19.62 N, the block could remain suspended in equilibrium within the water.
Throughout the calculation, density is expressed in kilograms per cubic metre, volume in cubic metres, and the final buoyant force correctly in newtons.
How it is examined
In Paper 1, objective questions often ask candidates to calculate buoyant force using Fb = ρVg, or to identify whether an object will float or sink from given density or weight values. In Paper 2, structured questions typically ask candidates to state Archimedes' principle, explain the law of flotation, and calculate buoyant force or the volume of fluid displaced for a described object.
In Paper 3, practical tasks commonly involve using a spring balance to weigh an object in air and then in water, asking candidates to determine the buoyant force from the difference in the two readings and relate it to the weight of water displaced.
A common mistake is using the total volume of an object that floats instead of only the submerged volume, or confusing the density of the fluid with the density of the object itself.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)