## Buoyancy - Buoyant Force and Upthrust

It is often observed that while swimming our body feels light or while taking out water from the well, the bucket feels lighter when it is partially or fully immersed in water. The reason behind this is that our body experiences forces from the downward direction or the opposite direction of the gravitational pull. This results in a decrease in weight. This is one of the reasons why the plastic balls float on the water instead of sinking in the water due to their weight.

The upward force exerted by a fluid opposes the weight of an object, immersed in the fluid. The pressure at the bottom of an object submerged in the fluid is always greater than at the top. The difference in the pressure of the fluid results in the net upward force on the object. This upward force is termed Buoyancy. It is necessary to understand density and relativity to completely understand the concept of Buoyancy.

The mass per unit volume of material is termed Density. The density is used to measure how tightly packed the matter is.

Density ρ = Mass/Volume=M/V

KG is the S.I unit or density, whereas, g is the C.G.S unit of density.

Relative Density: The ratio of the density of a substance to the ratio of the density of the water is termed the relative density or the specific gravity of a substance.

The relative density is measured as follows:

Relative Density = Density of a substance / Density of water

The relative density is the ratio of a substance having similar quantities; therefore, there is no unit for relative density.

Buoyancy is one of the main reasons why an object floats in water or fluid. The force exerted on fluid when an object is partly or fully immersed in the liquid can be termed Buoyancy. The differences in pressure on the opposite side of an object are the buoyant forces.

Newton (N) is the unit used to describe the buoyant force.

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**For example:** When a body is in water some amount of water is displaced in the water due to its weight. This amount of water is determined based on the density of an object which is related to the volume as well.

The center of Buoyancy is the point where the force is applied to the object.

### Buoyant Force/ Upthrust:

The upward force exerted by an object when an object is partly or fully immersed in a fluid is called the Buoyant Force. The buoyant force makes a body appear lighter when immersed in fluid partially or wholly.

An object tends to sink if the density of the object is greater than the density of the fluid it is submerged in. But if the density of the object is lower than the density of the liquid it is submerged in then the object will float. In other words, if the relative density of a substance is less than 1, the substance will float in water whereas, if the relative density of a substance is more than 1 then the substance will sink in the water.

### The Buoyant Force Formula is as Follows:

Fbuoyant = ρVfg

In this formula, pVg is the density of the displaced fluid multiplied by the volume of the displaced fluid. The density p -= m/V, therefore, m = pV. From this, we know that pVf is the mass of the displaced fluid.

We can also replace pVf with m.

Thus, Fbuoyant = mfg

The buoyant force depends on two important factors:

The density of fluid it is placed in.

The volume of the body.

There are various applications of the buoyant force. Some of the applications are given below:

### Submarine

The large ballast tank in the submarine is of great use to control its position and depth. The ballast tank allows the water to get into the submarine as it submerges in water and to make it weigh greater than the buoyant force.

### Hot Air Balloon

The buoyant force is used to raise and float the hot air balloon. The air in the atmosphere exerts a buoyant force on the object. The hot air balloon descends when the weight of the balloon is greater than the buoyant force. It becomes stable when the buoyant force and the weight of the hot air balloon are the same.

### Ship

The overall density of a ship is less than that of the sea, because of the hollow-like structure of the ship. The volume of the water displaced by the ship gives equal weight to the ship. The buoyant force is large to give support to the ship and make it float.

### Fish

Most of the fishes use the Archimedes Principle to swim in the water. The fish go up and down in the water and fill its air sac or the swim bladder with gases. The gases diffuse from their body and make their body lighter in weight. This helps the fish to go up in the water.

### Following are the Factors that Affect Buoyancy:

The volume of the fluid, the substance is placed in.

The density of the fluid.

Acceleration due to the gravitational force.

The mass of an immersed object and the density of an immersed object do not affect the buoyancy and the buoyant force. The overall depth of the object submerged does not affect the buoyant force. The deeper depth will not have any kind of effect on the buoyant force. The pressure at the top and bottom of the object will increase and decrease at the same rate when an object descends or ascends in the fluid. Therefore, the buoyant force remains unchanged even when the object goes deeper into the fluid. It is important to know the weight of the displaced fluid to find out the buoyant force of an object.

The buoyant force of an object = the weight of the fluid displaced by the object.

It is very essential to understand the buoyancy and the buoyant force to calculate and determine whether an object will sink or float when submerged in a fluid.

Example: A cubical shape 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.

Ans:

Let the side of the cubical iceberg be h.

The total volume of the iceberg =

The volume of the submerged portion is = (h -20) x

Now, For flotation, the weight of the iceberg = weight of the displaced water

The side of the iceberg is 164 cm.

Thus, the volume of the iceberg is 4.41

Answer: 4.41

## FAQs on Buoyancy

**1. What is Buoyancy in simple terms?**

We see many objects moving upwards when they are thrown into the water. Some objects float, some objects sink. The reason behind the floating of the objects like a plastic ball and a ship when they are in the water is buoyancy. It is the nature or tendency of the objects to float on liquids. We can see this similar force even in gases along with the liquids and this upward thrust is exerted when the objects are immersed in them. The principle behind this concept is opposite pressures of the object immersed and liquid in which it is immersed act upon each other and the stronger force persists. The buoyancy depends upon the factors like the volume of the liquid displaced by the object when immersed, the density of the fluid in which the object is immersed and the acceleration caused due to gravity. The best example to explain this concept is when you throw or try to immerse a plastic hollow ball into the beach water, it returns or raises and this is due to buoyancy.

**2. What are the three types of Buoyancy?**

Buoyancy is the force that leads to the floating of some objects. An object tends to rise when it is immersed in gas or liquids. This force is dependent on the volume of the fluid displaced when the object is immersed, acceleration caused due to gravity, the density of the fluid in which we immerse the object. Buoyancy force can be divided into 3 basic types:

Positive buoyancy is the reason why an object floats, when a lighter object is immersed in a heavier fluid or gas, the upthrust force is stronger for the liquid and hence the object floats. The best example is a ball immersed in the water, here, the ball is lighter than the water, and hence, the ball floats.

Another type is negative buoyancy, when you throw a metal piece into the water, it sinks in it and the reason behind this is negative buoyancy. When the object immersed has greater density than the fluid or the gas in which it is immersed, then the object sinks in the fluid and the reason behind this is the negative buoyancy.

The third and final type is the neutral buoyancy force, in this case, the weight or density of the object immersed is equal to the density of the fluid in which it is thrown. The best example to describe this concept is scuba divers who jump into the water.

**3. What are the examples of Buoyancy?**

Buoyancy is the force that causes some objects to float. Some objects tend to rise when it is immersed in a gas or liquid. This force is dependent on the following three factors, the volume of the fluid displaced when the object is immersed, acceleration caused due to gravity, the density of the fluid in which we immerse the object. Following are some of the examples of buoyancy:

It is the reason why fish, swimmers, submarines keep afloat on the water. Icebergs are very huge yet, they perfectly float on water due to buoyancy.

We see hot air balloons as huge as a small hut floating in the air, the air exerts some buoyancy force on the balloon, it floats as long as the balloon is lighter and comes down when it becomes heavier, it stays stationary when the relative density is equal for the balloon and the air.

A ship is built in such a way that it could be raised hugely but the base is flat and hollow having relatively lesser density than the water. So, the ship displaces the water exactly the necessary amount and hence it floats easily.

**4. What are the differences between Buoyancy and Upthrust?**

When we immerse an object in a gas or fluid, it either sinks or floats. If the object floats, the force which makes the object float is upthrust force, it is exerted by the liquid or gas on the object that we try to immerse and depending on the relative density of the object and the density of the fluid in which we immerse, the object sink or float and this phenomenon is called buoyancy. It is of three types namely, positive force, negative buoyancy, and neutral buoyancy.

**5. Why do ships float in water?**

Buoyancy is the force that leads to the floating of some objects. An object tends to rise when it is immersed in gas or liquids. This force is dependent on the volume of the fluid displaced when the object is immersed, acceleration caused due to gravity, the density of the fluid in which we immerse the object. A ship is built in such a way that it could be raised hugely but the base is flat and hollow having relatively lesser density than the water. So, the ship displaces the water exactly the necessary amount and hence it floats easily.