| English | Buoyant force |
|---|---|
| Bahasa Melayu | Daya apung |
| 中文 | 浮力 |
Definition
The upward force exerted by a fluid on an object immersed in it.
In exam terms
The buoyant force, also called upthrust, is the upward force a fluid exerts on an object immersed in it. It is a vector quantity, always acting upwards, and is measured in newtons (N).
By Archimedes' principle, the buoyant force equals the weight of the fluid displaced: F_b = ρ × V × g, where ρ is the fluid density in kg m⁻³, V is the volume of fluid displaced in m³, and g is the gravitational field strength in N kg⁻¹. For example, an object that displaces 0.002 m³ of water gives F_b = 1000 kg m⁻³ × 0.002 m³ × 9.81 N kg⁻¹ ≈ 19.6 N. The buoyant force arises because pressure in a fluid increases with depth, so the upward push on the base is greater than the downward push on the top.
Do not confuse it with weight
Do not confuse the buoyant force with the object's weight. Weight is the downward gravitational pull on the object (W = mg), while the buoyant force is the upward push of the fluid.
Whether the object floats or sinks depends on which is larger: it floats when the buoyant force can equal its weight, and sinks when weight is greater.
Also keep it separate from Archimedes' principle, which is the statement giving the value of the buoyant force. In Paper 2 you may be asked to state Archimedes' principle or to calculate the buoyant force using the weight of displaced fluid, and to compare it with the object's weight to decide floating or sinking.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)