Buoyant Force

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While swimming, we tend to feel lighter at some point of time. In the same manner, as one draws water from the well, the bucket feels lighter, when it is partially or fully immersed in water. Why do we feel this change in weight? When we immerse a body in water, or for that matter in any other liquid, it tends to experience an external force from the downward direction. This will be opposite to the direction of the gravitational pull of the planet and results in a decrease in weight of the body.

You would have noticed a plastic ball floating in water instead of sinking. On the other hand, some materials like a needle, sink in water as well. Let’s study the concepts of buoyancy and buoyant forces!

Buoyancy and Buoyant Force

Objects tend to float as a result of the buoyant force. It is basically the external force experienced by an object which is partially or fully immersed in water or any other fluid. Buoyancy can be defined as the upward force applied by the fluid on a body if it is either put in or submerged in the fluid. In short, buoyancy is the result of the pressure acting on the opposite sides of an object or body immersed in a static liquid. It is commonly known as ‘buoyant force’. Its SI unit is Newton (N).

Force of Buoyancy

buoyant force

When we try to submerge an object in a fluid, it tends to experience an upward force. The force applied by the fluid on the object is called the force of buoyancy. If an object is immersed in fluid, some amount of fluid gets displaced owing to its weight. The amount of fluid displaced is in tune with the object’s density, related to its volume. The measure of buoyant force is equal to the amount of fluid displaced by the material.

Centre of Buoyancy

buoyant force

The point of the object where the force of buoyancy is experienced is referred to as the center of buoyancy. The force of buoyancy is applied vertically, as a result the center of buoyancy is a point on the center of the gravity of the fluid which is displaced by the immersed object.

Few objects float in water while others sink

The pressure keeps fluctuating between water layers or any other liquid. Pressure on the bottom of water will be greater than pressure on its top. Because of the pressure difference between different layers, there will be a made-up force applied in the upward direction on the material immersed. As a result the submerged object will experience acceleration in the upward direction. 

The magnitude of the upward force will be equal to the pressure difference between the topmost and lowest layer. It is equivalent to the amount of water displaced by the immersed object as well. This causes the phenomenon of ‘floating’. If an object is supposed to float, it should be less dense than water. In case the density is more than that of water, it sinks.

Buoyancy and Temperature

The buoyant force will be inversely proportional to the temperature of the liquid in which the object is immersed in. While increasing the temperature of the fluid, its volume will increase and density will decrease. Hence, if the temperature of the liquid is increased, there will be a decrease in the upthrust or the buoyant force. In the same manner, if the temperature of the liquid decreases, the upthrust will increase.

Density and Relative Density

buoyant force

For understanding buoyancy better, we need to learn about density and relative density. 

‘Density’ of an object is its mass per unit volume. It is the measure of how tightly matter is packed inside an object. 

Density = ρ = Mass/Volume=M/V = kg/m3 (SI Unit)

Relative density of a substance is the ratio of its density to the density of water. Since relative density is the ratio of two similar quantities, it does not have a unit.

In case the relative density of a substance is less than one, it floats in water. On the other hand if its relative density is greater than one, it will sink in water.


Upthrust can be defined as the buoyant/upward force experienced by an object which is partially or wholly immersed in a fluid. As a result of buoyant force or upthrust, a body immersed partly or wholly in a liquid seems to be lighter. Basically, buoyant force/upthrust depends on two things, density of the fluid and the volume of the object immersed/amount of fluid displaced by it. 

Applications of Buoyancy

The phenomenon of buoyancy helps swimmers, fish submarines, and icebergs stay afloat. Here let us discuss some of the applications of buoyancy. 

Submarines: Large ballast tanks in submarines control their depth and also their position from the sea’s surface. Submarines submerge by allowing the water to come inside the ballast tank. As a result their weight becomes much greater than the buoyant force. 

Fish: A fish fills its air sacs with gasses in order to go up and down the surface of water. The gasses tend to diffuse from its body to the bladder, hence making it feel lighter. 

Hot Air Balloon: Our atmosphere is filled with air and it will exert buoyant force on all other objects. Hot air balloons rise and float in the air because of the very same reason. It descends as soon as its weight becomes greater than the buoyant force. The balloon becomes stationary if its weight equals the buoyant force.

Ship: The amount of water displaced by a ship has a weight equivalent to that of the ship. A ship is made hollow, its overall density is less than that of the seawater. The buoyant force acting on the ship is huge enough to support its weight.


Greek mathematician Archimedes discovered the laws related to buoyancy. These laws are known as Archimedes Principle. 

The word buoyancy is derived from the Spanish word ‘boyar’, which means float.

It is noticed that ships tend to float higher in seas and oceans which are more dense and cold than in seas which are warm and tropical. Ships will also float higher during the winter season. 

Check your knowledge

 Answer) If an object is either put inside a particular fluid or submerged in it, there tends to be an external and upward force which is applied to the body by the fluid. This phenomenon is called buoyancy.

 Answer) Newton (N). 

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