# Buoyant Force

## Introduction:

We often observe that our body feels lighter while taking a swim in the pool. Also, when drawing water from a well, the bucket feels lighter till it is partially or fully immersed in water. Have you ever wondered why this change in weight is felt? We can say, when immersed in water or any other fluid, the body experiences a force from the downward direction opposite to the direction of the gravitational pull, which is responsible for the decrease in its weight. It can be the reason why a ball of plastic floats in water rather than sinking to the base by its weight. But here too, we observe that some objects such as a plastic bottle float over water, while others such as a needle sink. In this section, we shall learn about the concept of buoyancy and buoyant force which gives a proper explanation of such observations.

### Buoyancy Definition:

Buoyancy is defined as an upward force exerted by a fluid that opposes the weight of an object immersed in a fluid. As we know that, the pressure in a fluid column increases with depth. Thus, the pressure at the bottom of an object submerged in the fluid is greater than that at the top. The difference in this pressure results in a net upward force on the object which we define as buoyancy.

In order to understand the concept of buoyancy properly, let us understand the concept of density and relative density.

Density: The density of a material is defined as its mass per unit volume. It’s a measurement of how tightly matter is packed together.

In numerical terms, density is defined as:

$Densit, \rho =\frac{Mass}{Volume}=\frac{M}{V}$

The S.I. unit of density is kg  and C.G.S. unit of density is g .

The density of water is 1000 kg  or 1g  at C.

Relative density: Relative density of a substance is defined as the ratio of density of the substance to the density of water at C. Relative density is also known as specific gravity of the substance.

$Relative\; Density = \frac{Density \: of \: a \: substance}{Density\: of \: water \: at\: 4^{\circ}C}$

Since relative density is a ratio of similar quantities, it has no unit.

### Buoyant Force / Upthrust:

Buoyant force is the upward force exerted on an object that is wholly or partly immersed in a fluid. It is due to buoyant force that a body submerged partially or wholly in a fluid appears to lose its weight i.e. appears to be lighter.

An object whose density is greater than that of the fluid in which it is submerged tends to sink. If the object is either less dense than the liquid or is shaped appropriately (as in a boat), the force can keep the object afloat. In terms of relative density, if relative density of a substance is less than one it floats in water and substances with relative density greater than 1 sink in water.

Buoyant force depends on:

1. i) The volume of the body immersed which is equal to the volume of fluid displaced.
2. ii) The density of fluid.

### Applications:

Submarine: A submarine has a large ballast tank, which is used to control its position and depth from the surface of the sea. A submarine submerges by letting water into the ballast tank so that its weight becomes greater than the buoyant force.

Hot Air Balloon: The atmosphere is filled with air that exerts buoyant force on any object. A hot air balloon rises and floats due to the buoyant force. It descends when the balloon’s weight is higher than the buoyant force. It becomes stationary when the weight equals the buoyant force.

Ship: A ship floats on the surface of the sea because the volume of water displaced by the ship is enough to have a weight equal to the weight of the ship. A ship is constructed in a way so that the shape is hollow, to make the overall density of the ship lesser than the sea water. Therefore, the buoyant force acting on the ship is large enough to support its weight.

Fish: Certain group of fishes uses Archimedes’ principle to go up and down the water. To go up to the surface, the fishes will fill its swim bladder (air sacs) with gases (clever isn’t it?). The gases diffuse from their own body to the bladder and thus making the body lighter. This enables the fishes to go up.