What is buoyancy? Concept note for CBSE Class 9 Physics

How do objects float in a fluid? The answer to this question is ‘buoyancy’. A buoyant force is a type of force that occurs in all gases and fluids. This force acts in an upward direction due to the presence of gravity. It is exerted opposite to the gravitational force. The buoyant force is found immersed in any object. Buoyancy can result from the pressure differences that act on an object’s opposite sides, which can be seen in a static fluid.

In this article, let us learn everything related to buoyancy and its other related concepts in detail, which will benefit the CBSE Class 9 students to obtain good marks in their Physics board exam.

Buoyant force

In Physics, the buoyant force is a force that is exerted on an object which is partially or completely immersed in a fluid. The other name for this upward force is called upthrust. Because of the presence of buoyant force, a body that is submerged fully or partly in a fluid will lose its weight. In simple terms, it will appear to be lighter.

There are a few factors of buoyant force. The following are some of them:

• The volume of the displaced fluid
• The density of the fluid
• Local acceleration due to the presence of gravity

An object will tend to sink if its density is greater than the fluid it has been immersed in. An object would float if its density is lesser than the density of the fluid or the liquid is appropriately shaped. For example, a boat floating in the sea could be considered in this case. A boat is a solid object with a lesser density than the sea it is floating on. This force is capable of keeping the objects afloat.

The main reason an object floats or sinks in a liquid substance is because of buoyancy. Buoyant forces are obtained when there are differences in pressure on the opposite sides of an object.

The unit used to describe buoyant force is newton (N).

It is essential to understand both the concepts of density and relativity to acquire the concept of buoyancy fully. 

Density can be defined as the mass per unit volume of material. The density usage is enormous as it could measure how tightly or loosely packed the matter or the substance is.

Density ρ = MassVolume = MV

kg is the unit for density meanwhile g is the C.G.S unit of density.

Relative density

In Class 9 Physics, the relative density is determined as the ratio of the substance’s density to the water’s density. The relative density of a substance is sometimes referred to as the specific gravity.

Below-given is the expression for relative density:

Relative density = Density of a substance density of water

Since the relative density is derived using the ratio of similar quantities, there will be a unit to measure it.

Example of buoyant force

Let us look at an example to make it easier for Physics students to understand this concept. Consider a body being thrown in water. Due to the body’s weight, some amount of water is displaced. This displacement is completely dependent on the object’s weight or the body. Furthermore, the amount of water displacement is determined by the density of the thrown object. It also depends on the volume.

Now, the centre of buoyancy is the point on the object where the force is applied. 

Why does a body sink or float in water?

To demonstrate it, we should first know that the water pressure at the bottom is much higher than at the top. If we go down the liquid, we can see that the number of overlying liquid layers that lay one over the other increases. So, in case someone considers a single liquid column, then it is viewed as a mixture of many overlying layers.

In addition, there will be a made-up force applied in an upward direction due to the difference in the pressure. This upthrust could lead to the object’s acceleration submerged in the upward direction. In general, this force always stays in the vertical direction, which is why it is called force or upthrust upward.

Apart from that, the magnitude of the force acting upwards can be seen as equivalent to the difference in the pressure of the last and top layers. It can also be seen as equivalent to the weight of the fluid displaced.

Therefore, as a consequence of that, floating occurs. The floating of an object is fully dependent on its depth or volume. For instance, an object would float higher on the top layer of the water surface if its density and weight are lesser. On the other hand, if the density and weight of an object are more, it will begin to sink.

Applications of buoyancy

There are so many uses due to the presence of the buoyancy concept. The Class 9 students should know the most common applications. They include swimming, ships, fish, icebergs, submarines, etc. Let us have a detailed understanding of some of the below-mentioned applications:

1. Hot air balloon

Our earth’s atmosphere is filled with air capable of exerting buoyant force on any object. Hot air balloons could rise and float due to the containment of buoyant force. Moreover, it will begin to descend when it loses air in the balloon. It could increase the weight of the balloon. Therefore, when the weight of the balloon becomes higher than that of the weight of the atmosphere, it will begin to go down. Furthermore, when the weight of the balloon equals the atmosphere’s weight, it will stay at a stationary position.

2. Submarine

Submarines are specifically designed to be used underwater. The large ballast tank present in the submarine can be of great use, which could help control its depth and position. The ballast tank would allow the water to get inside the submarine as it submerges in water. It will also make it weigh more than that of the buoyant force. So, that is how a submarine works using the buoyancy concept.

3. Ship

The volume of sea or ocean water displaced by the ship is essential in helping it float on the water’s surface. The volume of the displaced water is approximately equal to the ship’s weight. The ship is manufactured in a way that it is in hollow shape and lightweight compared to the seawater. The overall density of the ship has to be lesser than the overall density of the water it floats in. Therefore, it is why the buoyant force acting on the ship is larger enough to support its weight.

4. Fish

Most fishes utilise the principle of Archimedes for swimming in water. The fish can go up and down inside the water body. It could easily fill the swim bladder or the air sac with the help of gases. When the gases come out of their body, it will make the fish light in weight, which will help them float and go up. Fishes use gills, and fins to make this process happen.

Example problem

Let us have a look at this example problem in CBSE Class 9 Physics, to understand the buoyancy concept in a much deeper manner:

Problem: A cubical shaped large iceberg whose specific gravity is 0.9 is floating in the seawater. If the iceberg proportion above the sea level is 20 cm and the specific gravity of the seawater is 1.025, determine the volume of the iceberg.

To find:

The total volume of the iceberg

Solution:

Let us consider the cubical iceberg side as h,

Volume of the submerged portion = h – 20 x

The weight of the iceberg during floatation = weight of the displaced water

The side of the iceberg = 164 cm

Hence, the volume of the iceberg = 4.41

Answer: 4.41

Conclusion

To conclude, the CBSE Physics students have obtained all the information related to buoyancy. In this article, they have learnt about buoyant force in detail while looking at its example. The students are also taught about relative density. Moreover, we have seen a detailed explanation of how and why objects float or sink when it is inside the water body.

In addition to that, the students acquired knowledge on the applications of buoyancy and how it is being used in different objects. Finally, an example problem is given for the CBSE students to understand the whole concept thoroughly to be beneficial in their Physics board exam.

FAQs

1. Explain the types of buoyancy.

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

Positive buoyancy: It usually occurs when the immersed object is lighter than the displaced fluid. It is the main reason why the object floats.

Neutral buoyancy: It generally occurs when the immersed object’s weight is equal to the displaced fluid. An ideal equal of neutral buoyancy would be a dive taken by a scuba diver.

Negative buoyancy: It usually occurs when the immersed object has a higher density than the fluid displaced. It would possibly result in the object’s sinking.

2. Write some of the differences between buoyancy and upthrust.

Upon immersing a body inside a fluid or a gas would make it float or sink. While doing so, if the body floats, then the force that acts on it is the upthrust force. The gas or the liquid usually exerts this force on the body we have tried immersing. It also depends on the relative density of the body and the density of the fluid in which we immerse. As a result, the body is capable of either sinking or floating. This phenomenon is referred to as buoyancy. Furthermore, there are three major types of buoyancy. They are neutral, positive and negative buoyancy.

3. What is the rule for buoyancy?

The rule for buoyancy is explained using Archimedes’ principle. It is also known as the physical law of buoyancy. It was discovered by an ancient Greek inventor and mathematician named Archimedes. He stated that anybody either partly or entirely submerged in a fluid (liquid or gas) at rest can be acted upon by a buoyant or an upward force. Here, its magnitude is equal to the fluid’s weight.

4. Does buoyancy require gravity?

The buoyant force that occurs on a body is mainly because of its fluid pressure. However, without gravity, pressure will not exist, and thus there will be no buoyant force.

5. Mention the factors that affect buoyancy.

The following are the two main factors that could affect buoyancy:

• The volume of the solid immersed in the fluid.
• The density of the fluid in which the solid is immersed.