Buoyant Force

Buoyancy is the tendency of an object to float in a fluid. All liquids and gases in the presence of gravity exert an upward force known as the buoyant force on any object immersed in them. Buoyancy results from the differences in pressure acting on opposite sides of an object immersed in a static fluid. After reading this article, you will be able to explain the buoyant force and why fluids exert an upward buoyant force on submerged objects.

Table of Contents

What is a Buoyant Force?

The buoyant force is the upward force exerted on an object wholly or partly immersed in a fluid. This upward force is also called Upthrust. Due to the buoyant force, a body submerged partially or fully in a fluid appears to lose its weight, i.e. appears to be lighter.

Following factors affect buoyant force:

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

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 the relative density is less than one, it floats in water and substances with a relative density greater than one sink in water.

What causes Buoyant Force?

Buoyant Force

When an object is immersed in water or any other fluid, we observe that the object experiences a force from the downward direction opposite to the gravitational pull, which is responsible for the decrease in its weight. This upward force exerted by the fluid opposes the weight of an object immersed in a fluid. As we know, 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.

Demonstration of Buoyant Force

Buoyant Force

When we submerge an object in a fluid, an upward force is experienced by the object. The fluid applies this force on the object, which causes it to rise, and we call this force buoyant force. The magnitude of this force is precisely equal to the amount of weight of the liquid displaced.

The point where the buoyant force is applied or the point on the object where the force acts is termed the Center of Buoyancy.

Here is a video demonstrating the buoyant force or buoyancy with the help of animations.


Why does an object float or sink in water?

If one considers a single column of liquid, then it can be viewed as a concoction of the many overlying layers, one over the other, with fluctuating pressure. The pressure at the bottom of the liquid would be greater than at the top; this is because as we go down in the liquid, the number of layers of the liquid that are overlying, i.e. the layers one over the other, increases.

Owing to the difference in the pressure amid the layers, there tends to be a made-up force being applied to it in the upward direction. This force leads to the acceleration of the object that has been submerged in the upward direction. The force is always in the vertical direction.

It can also be said that the magnitude of the upward force is equivalent to the difference in the pressure of the topmost and the last layer and equivalent to the weight of the fluid displaced.

Floating is the consequence of the above concept. The object should be less dense than water, or else, if its density is greater, it will tend to sink.

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Applications of Buoyancy

It is due to buoyancy that human swimmers, fish, ships, and icebergs stay afloat. Some applications of buoyancy are given in the points below.

Submarine

Buoyancy makes it possible for swimmers, fishes, ships, and icebergs to stay afloat. Some of the prominent applications of buoyancy are given below:

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 seawater. Therefore, the buoyant force acting on the ship is large enough to support its weight.

Fish

A certain group of fishes uses Archimedes’ principle to go up and down the water. To go up to the surface, the fish 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 makes the body lighter. This enables the fish to go up.

Frequently Asked Questions – FAQs

Why is buoyant force important in swimming?

Buoyancy is important in swimming because it helps the swimmer to stay closer to the surface. This is because the pressure experienced by the swimmer under the water is more than the pressure experienced above. This is also the reason why swimmers can float on the surface of the water.

What causes buoyancy?

The pressure exerted by the fluid in which the object is immersed causes the buoyancy. Also, the buoyant force experienced by the object is always upwards because the pressure of the fluid increases with the depth.

What are the three types of buoyancy?

The three types of buoyancy are positive buoyancy, negative buoyancy, and neutral buoyancy.

Positive buoyancy is when the immersed object is lighter than the fluid displaced and this is the reason why the object floats.

Negative buoyancy is when the immersed object is denser than the fluid displaced which results in the sinking of the object.

Neutral buoyancy takes place when the weight of immersed object is equal to the fluid displaced. Dive taken by the scuba diver is an ideal example for neutral buoyancy.

What are the examples of buoyancy?

A boat or a ship floating in the water and the floating of cork in water are examples of buoyancy.

How are buoyancy and density related?

Buoyancy or the buoyant force is directly proportional to the density of the immersed fluid.

If you wish to learn more physics concepts with the help of interactive video lessons, download BYJU’S – The Learning App.

Test your knowledge on Buoyant Force

2 Comments

  1. excellent article
    thank you very much

  2. Great article. Well presented and easy to understand

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