When an object is submerged in a fluid, the fluid exerts an upward force called upthrust. It arises because pressure increases with depth, so the pressure on the bottom face is always greater than the pressure on the top face. Whether an object floats or sinks depends on how the upthrust compares with its weight.
What is upthrust and why does it occur?
Upthrust (also called buoyancy force) is the net upward force exerted by a fluid on any object submerged in it or partially submerged in it. It arises from the fact that fluid pressure increases with depth:
- The bottom surface of a submerged object is at a greater depth than the top surface.
- Therefore, the pressure acting on the bottom face is greater than the pressure acting on the top face.
- Both pressures act perpendicular to the surface; the net result of all the pressure forces is a single upward force — the upthrust.
This explanation is purely from fluid mechanics — the upthrust always acts upward regardless of the shape of the object, the fluid, or the object's weight.
What is Archimedes' principle?
Archimedes' principle states:
The upthrust acting on an object in a fluid is equal to the weight of fluid displaced by the object.
Displaced fluid is the fluid that the object "pushes out of the way" by occupying space. If a block of volume V is fully submerged in water (density ρ_water):
Upthrust = weight of displaced water = m_displaced × g = ρ_water × V × g
Units: ρ in kg/m³, V in m³, g = 10 N/kg (approximate) → upthrust in N.
Worked example: A metal cube with sides 0.10 m is fully submerged in water (density 1000 kg/m³). g = 10 N/kg.
Volume of cube = 0.10 × 0.10 × 0.10 = 0.001 m³ Upthrust = ρ_water × V × g = 1000 × 0.001 × 10 = 10 N
If the cube weighs 27 N, the net downward force = 27 − 10 = 17 N → it sinks. If the cube weighs 8 N, the net force = 8 − 10 = −2 N (net upward) → it rises to the surface.
What determines whether an object floats or sinks?
Compare the upthrust with the object's weight:
| Condition | Net force | What happens |
|---|---|---|
| Weight > upthrust (when fully submerged) | Net downward | Object sinks to the bottom |
| Weight = upthrust | Zero net force | Object floats fully submerged (neutrally buoyant) |
| Weight < upthrust (when fully submerged) | Net upward | Object rises; floats partially submerged at the surface |
Floating: When an object floats, it displaces enough fluid so that the upthrust exactly equals its weight. A floating object is always in equilibrium: weight downward = upthrust upward.
Density shortcut: Comparing densities gives the same answer more quickly. If the object's average density is less than the fluid's density → it floats. If greater → it sinks. If equal → it is neutrally buoyant.
Wood floats on water because wood's density (~500–900 kg/m³) is less than water's (1000 kg/m³). Steel sinks because steel's density (~7800 kg/m³) is much greater.
How can a steel ship float?
A solid block of steel sinks — its density is about 7.8 times that of water. Yet a steel ship floats. The explanation is that a ship is not a solid block of steel — it is a hollow structure enclosing a large volume of air.
The average density of the whole ship (steel + air + cargo + everything inside) is less than the density of water. When the ship is lowered into water, it sinks until the upthrust from the displaced water equals the total weight. Because the average density of the ship is less than water, this equilibrium is reached before the ship is fully submerged — the ship floats with its hull partly above the waterline.
A submarine uses ballast tanks — it floods them with water (increasing average density → sinks) or blows them out with compressed air (decreasing average density → rises). The buoyancy can be precisely controlled.
How does upthrust apply to gases?
Upthrust works in gases (which are fluids) too, though air is much less dense than water:
- Air density ≈ 1.2 kg/m³ vs water density ≈ 1000 kg/m³ → upthrust in air is about 800 times smaller than in water for the same volume.
- A helium-filled balloon rises because helium is less dense than air → upthrust (weight of displaced air) > weight of balloon → net upward force.
- Objects on Earth experience a small upthrust from air — this is why very precise measurements of mass must correct for air buoyancy.
Frequently asked questions
What is upthrust in KS3 physics?
Upthrust is the upward force that a fluid (liquid or gas) exerts on any object immersed in it. It arises because fluid pressure increases with depth, so the pressure on the bottom of an object is greater than on the top, producing a net upward push. According to Archimedes' principle, the upthrust equals the weight of fluid displaced by the object. Whether the object floats or sinks depends on whether the upthrust is greater than, equal to, or less than the object's weight.
Why does a heavy ship float while a small stone sinks?
A stone is solid and dense — its average density is greater than water — so even when fully submerged, the upthrust is less than its weight and it sinks. A ship is a hollow structure filled largely with air; its average density (total mass divided by total volume, including the air inside) is less than water. When placed in water, the ship sinks until the weight of water displaced equals the ship's total weight — at which point it floats, with part of the hull above the waterline. The key factor is average density, not total mass.
What is Archimedes' principle and why is it useful?
Archimedes' principle states that the upthrust on an object equals the weight of fluid it displaces. It is useful because it allows you to calculate the exact upthrust without needing to analyse all the pressure forces individually. You just need the volume of the submerged object and the density of the fluid. If the object floats, the upthrust equals the weight — meaning the weight of displaced fluid equals the weight of the floating object, which gives a quick way to find either weight or volume.
How do submarines use upthrust to dive and surface?
Submarines control their average density using ballast tanks. To dive, seawater is let into the tanks, increasing the total mass of the submarine while keeping the volume roughly the same — the average density rises above seawater, so weight exceeds upthrust and the submarine sinks. To surface, compressed air is forced into the tanks, expelling the water — the average density drops below seawater, so upthrust exceeds weight and the submarine rises. At any chosen depth, the tanks can be set so weight exactly equals upthrust and the submarine hovers neutrally buoyant.
For Socratic KS3 physics with Professor Newton — predicting floating from pressure differences before stating Archimedes' principle, so the rule makes intuitive sense — visit aitutors.me.