If you’ve ever tried to push a piece of cock underwater, you’ve probably noticed it pushes back up, unlike an iron ball that immediately sinks. What’s happening here? These phenomena occur due to buoyancy, the tendency of an object to float in any fluid.

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Objects stay afloat because of the buoyant force. Thus, when you push a cork underwater, it rises back to the surface due to the upward force known as Upthrust.

Buoyancy 2



What is buoyant force

The buoyant force exerts an upward force on an object wholly or partly immersed in a fluid. Consequently, differences in fluid pressure create the buoyant force.


When a fluid submerges an object, it experiences pressure from all directions. The horizontal pressures on the sides cancel each other out. Since pressure in a fluid increase with depth, the pressure at the bottom of the object is greater than at the top. This pressure difference creates a net upward force known as the buoyant force.

Buoyancy 3


Buoyancy and Gravity

While buoyant force pushes up on an object, gravitational force (weight) pulls it down. The relative strength of these forces determines whether an object sinks or floats.

There are three types of buoyancy: positive, negative, and neutral.

  • Positive buoyancy: Buoyant force > object’s weight, causing it to float.
  • Negative buoyancy: Buoyant force < object’s weight, causing it to sink.
  • Neutral buoyancy: Buoyant force = object’s weight, keeping it suspended.



Now, you might wonder: Why does an iron nail, with a lighter mass, sink in water, while a ship, made of iron and heavier, floats? This brings us to the Archimedes’ principle.


Archimedes Principle

Consider when a submerged object is removed from a fluid, it displaces fluid with weight Wf, which is supported by the surrounding fluid. So, the buoyant force equals Wf, the weight of the displaced fluid. This concept forms the basis of Archimedes’ principle.B5


Therefore, the iron nail sinks due to its smaller volume, displacing less water and resulting in a smaller buoyant force. In contrast, ships float because of their larger volume, displacing more water and experiencing a greater buoyant force.

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Archimedes’ Principle states that the buoyant force on an object equals the weight of the fluid it displaces. Specifically, it can be mathematically written as:

FB = Wf


  • FB is the buoyant force
  • Wf is the weight of fluid displaced


Given mass = density x volume, we can rearrange the formula to get:

FB = ρfVfg


  • FB is the buoyant force
  • ρf is the density the fluid
  • Vf is the volume of fluid displaced
  • g is the acceleration due to gravity



Hence, factors affecting the buoyant force are…

  • the density of the fluid
  • the volume of the fluid displaced
  • the acceleration due to gravity


The buoyant force is not affected by…

  • the mass of the immersed object
  • the density of the immersed object


Buoyancy and Density

Buoyancy is closely linked to density, which is the ratio of an object’s mass to its volume. Whether an object floats or sinks, and how deep it does so, depends on its average density relative to the fluid’s density.Equation


An empty ship, having lower average density, stays less submerged than a loaded one because it’s pushed up by a stronger upward force from the denser surrounding fluid.

Buoyancy B8

Do you recall the sinking of the RMS Titanic in 1912? The Titanic, like other ships, had large, hollow steel hulls, which lowered its average density. However, when it struck the iceberg, the hull broke, allowing water to enter the ship, displacing air and increasing its density beyond that of the surrounding water. Consequently, it sank to the ocean floor.



Applications of Buoyancy

Buoyancy is related to our everyday life. Let’s explore some examples.


Fish, Submarines

Fish achieve buoyancy through an organ called swim bladder, which is basically like an air-inflated balloon that can expand and contract depending on how much gas is inside. It expands as they rise and contracts as they dive deeper, aiding their movement.

Submarines work in a similar way as the swim bladder. Although instead of a bladder, the submarine uses a ballast tank that allow the crew to control the depth of the vessel.


Hot Air Balloon

The atmosphere exerts buoyant force on objects. Heating the air inside the balloon makes it less dense than the surrounding air, pushing the hot air balloon up. To descend, the gas heaters are turned off, and the air cools. Consequently, opening a vent at the top allows cooler air to enter, increasing the air’s density and causing the balloon to slowly descend.Buoyancy 10


Why Does an Object Float or Sink in Water?

Buoyancy 10

FB > Wobject FB = Wobject FB < Wobject
ρobject < ρfluid object floats object floats
(partly immersed)
ρobject = ρfluid

object suspends

(fully immersed)

ρobject > ρfluid object sinks

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