Buoyant Force

All of us have experienced our bodies feeling lighter while swimming at some point in time or another. Furthermore, while drawing water from the well, the bucket feels lighter if it is either partially or fully immersed in water. Have you ever thought about the fact why we feel a change in weight? Well, when we immerse a body or an object in water or any other fluid, it experiences an external force from the downward direction, which is opposite to the direction of the gravitational pull of the earth and is responsible for the decrease in the weight of the body. 

The same is the reason behind a plastic ball floating in water instead of sinking to the base of the water by its weight. However, in this case, we have also observed that some objects like a needle sink in water as well. So, let's make ourselves familiar with the concepts of buoyancy and buoyant forces and seek an adequate explanation of all these observations.

Understanding the Term Buoyancy and Buoyant Force 

The term 'Buoyancy' refers to the force that causes the objects to float. To be specific, it is the external force experienced by an object that is either partly or fully immersed in water or any other fluid. We can also define buoyancy as the upward force applied by the fluid on a body or object when it is either put in or submerged in the fluid. The phenomenon of buoyancy is caused by the pressure acting on the opposite sides of an object or body immersed in a static fluid. It is also commonly referred to as 'buoyant force' (the upward force experienced by a body or an object when it is immersed in a fluid), so we can say that buoyancy is the phenomenon caused due to buoyant force. 

Newton (N), the unit of Force (F), is the unit of the Buoyant Force as well.

What is the Force of Buoyancy

An upward force is experienced by an object or a body when we submerge it in a fluid. This force applied by the fluid on the object, which causes it to come upwards is, what we call the Force of Buoyancy. Whenever we immerse an object in a fluid, it displaces some amount of the fluid owing to its weight. The amount of fluid displaced corresponds to the object's density, which, in turn, relates to its volume. The scale or measure of the buoyant force is also precisely equal to the amount of fluid displaced by the object.

What is the Centre of Buoyancy?

The point on the object where it experiences the force of buoyancy is what we refer to as the Centre of Buoyancy. It is essential to understand that the force of buoyancy is applied vertically, due to which, the centre of buoyancy is a point on the centre of the gravity of the fluid displaced by the immersed object.

Why do Few Objects float in Water while Others Sink?

In between the layers of water or any other liquid, the pressure keeps fluctuating. The pressure on the bottom of the water is greater than the pressure on its top. As a result of this pressure difference amid the different layers, there tends to be a made-up force applied in the upward direction on the object immersed. Due to this force, the submerged object experiences acceleration in the upward direction.

We can also say that the magnitude of the upward force is equal to the pressure difference between the topmost and lowest layer and is also equivalent to the amount of water displaced by the immersed object. The consequence of this concept gives rise to the phenomenon of 'floating.' For an object to float, it should be less dense than water, and if its density is more than that of water, it will sink.

The Relationship between Buoyancy and Temperature

The buoyant force tends to be inversely proportional to the temperature of the fluid or liquid in which the object or body is immersed in. This is because, with an increase in the temperature of the fluid, there will also be an increase in its volume and a decrease in its density. Therefore, if the temperature of the liquid is increased, the upthrust or buoyant force will decrease. Similarly, if the temperature of the liquid is decreased, then the upthrust will end up increasing. 

The Concepts of Density and Relative Density

For having a better understanding of the concepts of buoyancy, we need to know about density and relative density.

The 'Density' of a material refers to its mass per unit volume. To be specific, it is the measure of how tightly matter is packed inside a material. The formula for density is as follows:

\[\text{Density} = \rho = \frac{ \text{Mass}}{ \text{Volume}} = \frac{M}{V}\] = kg/m3 (SI Unit)

A substance's relative density (also known as the specific gravity of a substance) is the ratio of its density to the density of water. The relative density is the ratio of two similar quantities; hence, it has no unit.

If the relative density of a substance is less than one, it will float in water, and if its relative density is greater than one, it will sink in water.

What is Upthrust?

The buoyant force or the upward force, which an object immersed partially or wholly in a fluid experience, is also known as 'Upthrust’. Due to the buoyant force or upthrust, a body or an object immersed partly or wholly in a fluid appears to be lighter. As discussed earlier, the buoyant force or upthrust depends on two things, namely, the density of the fluid, and the volume of the object immersed or the amount of fluid displaced by it.

What are the Applications of Buoyancy? 

Due to the phenomenon of buoyancy, swimmers, fish, submarines, and icebergs stay afloat. Let us discuss some of the applications of buoyancy using a few examples mentioned below:

Submarines - There is a large ballast tank in submarines that controls their depth and position from the sea's surface. Submarines submerge by letting the water come inside the ballast tank so that their weight becomes much greater than the buoyant force.

Fish - For going up and down the surface of the water, a fish fills its air sacs with gases, which, in turn, diffuse from its body to the bladder; thus, making it feel lighter. 

Hot Air Balloon - As the atmosphere is filled with air, it exerts a buoyant force on every other object. Due to this buoyant force, a hot air balloon rises and floats in the air. It begins to descend as soon as its weight becomes greater than the buoyant force and becomes stationary when its weight equals the buoyant force.

Ship - A ship floats on the surface of the water as the amount of water it displaces is sufficient to have a weight equivalent to the ship's weight. A ship is usually made hollow so that its overall density is less than that of the seawater. Hence, the buoyant force acting on the ship is pretty large to support its weight.

Interesting Facts

The laws related to buoyancy were discovered by Archimedes who was a Greek Mathematician, and these laws have come to be known as The Archimedes Principle. 

The word buoyancy comes from a Spanish word called “boyar”, which means float. 

Ships will float higher in seas and oceans that are more dense and cold than they would in seas that are more warm and tropical. Ships also tend to float higher during the winter season.