Buoyancy: Buoyant Force of Liquid and Gas with Example

Buoyancy: Students curious about Buoyancy meaning must know that Buoyancy is an upward force exerted by the fluid. It is the reason why objects float on water. All liquids and gases exert an upward force known as the buoyant force on any object immersed in them. An iron ball may sink in seawater, but a huge ship made of iron will float on seawater. Have you ever wondered; that an egg that may float on saltwater may sink in freshwater! Intriguing isn’t it?

Buoyancy is caused by pressure differences acting on opposing sides of an item immersed in a static fluid. Pressure will increase as you go deeper into a fluid, the force exerted by pressure on the top of the can of beans will be smaller than the force exerted by pressure on the bottom of the can. In this article, we will talk about the buoyancy meaning, positive buoyancy, neutral buoyancy, buoyancy and floatation, and the principle of buoyant force with some examples.

Buoyancy Meaning

Buoyancy is a special property of fluids to make objects immersed in them lighter. Fluids are those substances that can flow. Liquids and gases are classified as fluids. When we immerse an object in a fluid, it experiences an upward force that makes it lighter. This property of fluids is termed buoyancy.

Buoyant Force Definition

The upward force exerted on an object immersed entirely or partially in a fluid is called Buoyant Force. This upward force is called Upthrust.

Due to the buoyancy of the water, we can swim in it easily. If the water is salty, it becomes even easier for us to swim in it. Buoyancy makes it easier to lift a heavy object when it is lying underwater than when it is lying on the ground. We will study this special property of fluids in detail.

Buoyancy in Fluid: Density Buoyancy

Buoyancy makes objects seem lighter when immersed in fluids. In such a case, apart from the weight of the object which acts in a downward direction, the object will experience a force due to the fluid. Thus, buoyancy is defined as under:
“When a body is partially or completely immersed in a fluid, it experiences an upward force on it. This property of fluids to exert the upward force on objects immersed in them is known as buoyancy.” Studying buoyancy in detail will give us a clear idea of how objects float or sink in fluids, the factors affecting them and their applications in day-to-day life. Students can check Archimedes’ principle to understand the upward buoyant force.

Buoyancy Formula

What is Neutral Buoyancy? Buoyancy is the reason that an object immersed in fluid experiences an upward force on it. This upward force is known as upthrust or buoyant force.

The buoyant force  \(\left( {{F_B}} \right)\) can be calculated using the formula,
Where, \({F_B} = V\rho g\)
\(V\) is the volume of the object immersed in the fluid or the volume of the fluid displaced by the object
\(\rho\) is the density of the fluid
\(g\) is the acceleration due to gravity

Unit

Buoyancy makes the objects immersed in fluids experience a buoyant force. So, the unit of buoyancy is the same as the unit of force.
In SI system of units, buoyancy is measured in newton \(\left( {\rm{N}} \right).\)
In the CGS system of units, buoyancy is measured using dyne.

Buoyant Force

Buoyancy is the property of the fluids to exert an upward force on the objects immersed in them. This upward force is known as buoyant force. The other name used for buoyant force is upthrust, acting on the object in the upward direction that is too perpendicular to the surface experiencing it.

So, the buoyant force is the upward force an object experiences when it is immersed completely or partly in a fluid. This force acts in a direction opposite to the direction of gravitational force responsible for the object’s weight. Thus, buoyant and gravitational forces act on an object in opposite directions.

When we calculate the net force acting on the object while it is immersed in a fluid, we must subtract the buoyant force from the weight of the object.
The actual weight of the object is measured when the object is in the air. When the object is immersed in a fluid (generally liquid), it will become lighter due to the action of the buoyant force exerted by the fluid. So, the weight of the object, in this case, is its apparent weight which will be always less than its actual weight.

When the apparent weight of the object becomes zero, then the object will float on the fluid and will experience weightlessness. This is when the buoyant force experienced by the object is equal to the weight of the object. This phenomenon is well explained by the Archimedes principle.

Principle of Buoyancy

The principle given by Archimedes can well understand buoyancy. It states that “When a body is immersed partially or wholly in a liquid at rest, it experiences an upthrust which is equal to the weight of the liquid displaced. The apparent loss in weight of the body is equal to the upthrust on the body.”

According to the principle, when a body is immersed in a liquid, it displaces some fluid. The liquid displaced is equal to the volume of the body immersed in the liquid.

The weight of the liquid displaced by the body immersed in it is equal to the upthrust experienced by the body while being immersed in the liquid.

And the apparent weight of the body will be the difference between its actual weight and the upthrust experienced by it.

Factors Affecting the Buoyant Force

Buoyancy is due to the interaction of the fluid with the object immersed in it. When fully or partially immersed in a liquid, it experiences upthrust or buoyant force. Its buoyancy is the property of exerting buoyant force on the objects immersed in a fluid. So, the buoyancy of a liquid depends on the factors affecting the buoyant force it exerts on the object immersed in it. They are:

1. The volume of the object immersed in the fluid: The buoyant force exerted on an object immersed in a fluid is directly proportional to the volume of the object immersed in it. The more the volume of the object immersed in the fluid, the more will be the buoyant force acting on it and vice versa.
2. The density of the fluid: The buoyant force is directly proportional to the density of the fluid. The higher the density of the fluid in which the object is immersed, the more will be the buoyant force experienced by the object. Due to this reason, we feel lighter in seawater as compared to freshwater and it is called density buoyancy
3. The acceleration due to gravity: The buoyant force is directly proportional to the value of the acceleration due to gravity.

Types of Buoyancy: Positive Buoyancy, Neutral Buoyancy & Negative Buoyancy

Based on whether the object floats or sinks and if it floats or is fully immersed or partially immersed, buoyancy is classified. In a nutshell buoyancy and floatation define the type of buoyancy.

1. Negative Buoyancy: The object sinks into the fluid in negative buoyancy. This is the case when the weight of the fluid displaced by the body is less than the body’s weight. So, the apparent weight of the body while being immersed in the liquid is more than zero.
2. Positive Buoyancy: In positive buoyancy, the object floats on the fluid and is partially immersed in it. This is the case when the weight of the fluid displaced by the body is equal to the body’s weight. So, the apparent weight of the body while being immersed in the fluid is zero.
3. Neutral Buoyancy: The object floats on the fluid and is completely immersed in it in neutral buoyancy. This is the case when the weight of the fluid displaced by the body is equal to the body’s weight. So, the apparent weight of the body while being immersed in the fluid is zero.

Applications of Buoyancy

Buoyancy finds many applications in our day-to-day life, especially during our interactions with fluids. Some of the applications are as under:

1. The submarines are designed keeping in mind the concept of buoyancy. It can float on as well as sink in water. This is done by filling the submarine with air thereby establishing positive buoyancy which enables it to float. Similarly, it fills in water to establish negative buoyancy thereby enabling it to sink in water.

2. A Ship made of iron floats on water whereas a ball made of iron sinks in it. The ship floats by displacing water equal to its weight, thereby getting enough buoyant force to float on water.

3. We can swim due to buoyancy. While being in the swimming pool or pond, we displace water equal to our body weight thereby getting help from the water itself to float on it and swim.

4. The life jackets help the user in open water bodies to float on them thereby preventing drowning. Again, this enables to displace water equal to the weight of the user to get enough upthrust for floating on water.

5. The hot air balloons are filled with air that is having a density lower than that of the surrounding air. This helps it to displace air equal to its weight and thereby receive upthrust for its upward movement.

Solved Examples

Example 1
A cubical block is dipped completely in water. Each edge of the block is \(1 \;{\rm{cm}}\) in length. Find the buoyant force acting on the cubical block. Take \(g = 10\;{\rm{m\;}}{{\rm{s}}^{ – 2}}.\)

Solution: Given that,
The volume of the cubical block is \(V = {\left( {{\rm{1}}\,{\rm{cm}}} \right)^{\rm{3}}}{\rm{ = }}{\left( {{\rm{0}}.{\rm{01}}\,{\rm{m}}} \right)^{\rm{3}}}{\rm{ = 1}}{{\rm{0}}^{{\rm{ – 6}}}}\,{{\rm{m}}^{\rm{3}}}\)
The density of water is \(\rho  = 1000\;{\rm{kg\;}}{{\rm{m}}^{ – 3}}\)
The acceleration due to gravity is \(g = 10\;{\rm{m\;}}{{\rm{s}}^{ – 2}}\)
The buoyant force acting on the cubical block is \({F_B} = V\rho g = {10^{ – 6}} \times 1000 \times 10 = {10^{ – 2}}\;{\rm{N}}.\)
Thus, the buoyant force exerted by water on the block is \({10^{ – 2}}\;{\rm{N}}.\)

Example 2: A body of \(5\;{\rm{kg}}\) floats on a liquid. What is the buoyant force on the body? Take \(g = 10\;{\rm{m\;}}{{\rm{s}}^{ – 2}}\)

Solution: Given that,
The mass of the body is \(m = 5\;{\rm{kg}}\)
The acceleration due to gravity is \(g = 10\; {\rm{ m\;}}{{\rm{s}}^{{\rm{ – 2}}}}\)
Since the body is floating on the liquid, so it would be experiencing a buoyant force equal to its weight.
The weight of the body is \(W = mg = 5 \times 10 = 50\;{\rm{N}}\)
So, the buoyant force on the body is \({F_B} = W = 50\;{\rm{N}}\)
Hence, the buoyant force on the body is \(50\;{\rm{N}}{\rm{.}}\)

Conclusion

Hope this article would have helped you understand the concept of buoyancy in detail, along with its types, formula and units. We gained some understanding of the principle of buoyancy, factors affecting it as well as its applications in real-life situations. Finally, we are also optimistic that this article would have helped you distinguish buoyant force from buoyancy.

FAQs

Check out frequently asked questions related to Buoyancy.

Q.1. What is buoyant force?

Ans: The buoyant force is the upward force exerted by the fluid on the objects immersed in it.

Q.2. What causes buoyancy?

Ans: Buoyancy is the property of fluids to exert an upward force on the objects immersed in them.

Q.3. How is buoyancy different from buoyant force?

Ans: Buoyancy is the property of fluids to exert an upward force on the objects immersed in them. This upward force is named upthrust or buoyant force. So, buoyancy is the property of the fluids to exert buoyant force, and buoyant force is the upward force experienced by objects immersed in the fluids.

Q.4. What is the formula of buoyancy? 

Ans: The buoyancy depends on the buoyant force it exerts on the objects immersed in it and is calculated by using the formula, \({F_B} = V\rho g.\)

Q.5. How is buoyancy related to density?

Ans: The buoyancy of any fluid depends directly on its density. The higher the density of the fluid, the more will be the buoyant force it exerts on the objects immersed in it.