Buoyant Force - Definition, Formula, Applications, FAQs

Q: It is given that, the density of an object placed on the surface of water is 10 kgm-3 and the density of water is 10-3 kgm-3. What will be the relative density of the object?

Given : Density of object = 10 kgm -3 Density of water = 10 -3 kgm -3 To find : Relative density of object Formula for Relative density ? Relative density =Density of object/Density of water =10kgm -3 /10-3kgm -3 =0.01 Hence, relative density of object is 0.01

Q: It is given that the volume of a 500 g packet is 350 cm3. What do you think, the packet will float or sink in water if the given density of water is 1 gcm-3 ?

Given : Volume of packet = 350 cm 3 Weight of the packet = 500 g Density of water = 1 gcm -3 To find : Density of packet, in order to compare it with density of water and see whether the packet will float or sink Density of packet = Weight of packetVolume of packet = 500g350cm 3 = 1.428 gcm -3 Therefore, the packet will sink because the density of the packet is greater than the density of water.

Buoyant Force - Definition, Formula, Applications, FAQs

Edited By Vishal kumar | Updated on Jul 02, 2025 04:25 PM IST

We all know that things float on the surface of water or any other liquid, but have you ever thought why? Why when we swim in a swimming pool, our body feels lighter than usual? Have you ever wondered why a ship made of iron and steel metals floats on the surface of water whereas a small iron nail sink in the water? Well, all these questions can be answered by taking buoyancy and buoyant force in consideration. Let us understand what is buoyancy by taking some common examples in the given article below.

This Story also Contains

What is Buoyancy?
What is Buoyant Force?
What is Upthrust Force?
Si unit of Buoyant Force
Buoyant Force Formula
Relative Density
Applications of Buoyancy

Buoyant Force - Definition, Formula, Applications, FAQs

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What is Buoyancy?

When we place an empty plastic bottle on the surface of water, it floats. We all know that gravitational force acts on everything and pulls it downwards. So what is it that keeps the plastic bottle afloat on the surface of water? The water exerts an anti gravitational force on the plastic bottle and hence causing it to float. This phenomenon which causes to float on the surface of water is known as buoyancy.

What is Buoyant Force?

Buoyancy Force Definition: Buoyant force is defined as the upward force exerted by water or any other liquid opposite to the gravity which makes an object to float on the surface of it.

In the above example, the plastic bottle floats as the buoyant force exerted by the water is greater than its weight which is nothing but the gravitational force exerted by the earth. If we want the plastic bottle to be completely immersed into the water, then we need to apply an external downward force which is equal to the gravitational force in order to balance it.

All objects experience a buoyant force when they are immersed fully or partially in a fluid. The magnitude of the buoyant force depends upon the density of the fluid in which the object is immersed.

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What is Upthrust Force?

The buoyant force is exerted in the upward direction, opposite to the gravitational force hence it is also known as the Upthrust force.

Si unit of Buoyant Force

As buoyant force is also a type of force, the SI unit of it is Newton (N)

Buoyant Force Formula

The formula for calculating buoyant force is

$$
F_b=-\rho g V
$$

$F_b=$ buoyant force
$\rho=$ fluid density
$g$ = acceleration due to gravity
$V$ = fluid volume

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Why objects float or sink when placed on the surface of water?

Whenever we talk about buoyant force or buoyancy, this question always arises why do some objects float on the surface of the water while some objects sink? When water exerts force on every object placed on the surface of it then why do some objects float and some do not? Let's resolve this query by taking a common example.

If we place an iron nail and a cork on the surface of water and both are of equal masses, we observe that the piece of cork floats on the surface of the water whereas the iron nail sinks even if the masses of both of them are same.

Iron nail and the cork placed on the surface of water

This is because their densities are different. The density of the cork is less than the density of the water which means, the buoyant force exerted by the water on the cork is greater then the weight of the cork which is nothing but gravity hence it floats. Whereas in the case of iron nail, the density of the iron nail is greater than the density of water which means that the buoyant force exerted by the water on the nail is less then its weight hence it sinks.

Therefore, the object placed on the surface of water will float or sinks completely depend on the density of the object as compared to the density of the water.

Related Topics,

Archimedes’ Principle

Archimedes’ Principle is a fundamental law of buoyancy of fluids discovered by a Greek scientist named Archimedes and it is stated as follows:

If we fully or partially immersed a body in a fluid, the upward force experienced by the body is equal to the weight of the fluid displaced by it when it is immersed in the fluid.

This principle explains that when a body is immersed fully or partially in water or in any other liquid, the amount of buoyant force exerted by the liquid and experienced by the object is equal to the fluid displaced by it in weight. Archimede's principle has many applications from designing ships and submarines to determining the density of a liquid.

Relative Density

In order to calculate buoyancy more accurately and also to determine the purity of a substance, the concept of relative density is necessary to understand.

Relative density of any substance is defined as the ratio of density of a substance to the density of water.

Relative density=Density of a substance/Density of water

Since, It is a ratio of same quantities, relative density has no unit.

Applications of Buoyancy

There are several applications of buoyancy. Some of the common natural and human made applications of buoyancy are stated below :

Ship ? Did you ever thought why ship float on water even after having such a high weight? This is because the density of ship less as compare to water due to a hollow-like structure. Hence, according to Archimedes principle the buoyant force exerted by water on it is quite high to support the ship and make it float.

Submarine ? Submarines are provided with a large ballast tank which gets filled up when the submarine submerge in the water which increases its weight then the buoyant force and when the submarine needs to get out of the water, the tank releases its water to make it lighter then the buoyant force.

Fishes ? Fishes have air sacks which they fill in order to increase their weight and move deep inside the water and when they need to move at the surface of water, they empty the air sacks which decreases their weight and the buoyant force automatically move them upward.

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Frequently Asked Questions (FAQs)

1. What is buoyant force? Give its formula and SI unit.

Buoyant force is defined as the upward force exerted by a liquid on an object placed on the surface of it. As buoyant force is exerted by water in upward direction it is also known as Upthrust force. It’s SI unit is Newton (N) and it’s formula is given by

F_buoyant =ρgV

2. Why some objects float when we place them on the surface of water while some sink?

An object will float or sink when placed on the surface of water completely depends on the density of object and density of water. If the density of object is greater than the density of water, it will sink and if the density of object is less than the density of water, it floats.

3. It is given that, the density of an object placed on the surface of water is 10 kgm-3 and the density of water is 10-3 kgm-3. What will be the relative density of the object?

Given :

Density of object = 10 kgm^-3

Density of water = 10^-3 kgm^-3

To find : Relative density of object

Formula for Relative density ? Relative density =Density of object/Density of water

=10kgm^-3/10-3kgm^-3

=0.01

Hence, relative density of object is 0.01

4. A person releases a plastic block under water. The block immediately come up to the surface of water. Why?

A block of plastic released under water come up to the surface of water because the density of block is less then the density of water, which means that the buoyant force exerted by the water is greater than the weight of plastic block hence it come up to the surface.

5. It is given that the volume of a 500 g packet is 350 cm3. What do you think, the packet will float or sink in water if the given density of water is 1 gcm-3 ?

Given :

Volume of packet = 350 cm³

Weight of the packet = 500 g

Density of water = 1 gcm^-3

To find : Density of packet, in order to compare it with density of water and see whether the packet will float or sink

Density of packet = Weight of packetVolume of packet

= 500g350cm³

= 1.428 gcm^-3

Therefore, the packet will sink because the density of the packet is greater than the density of water.

6. Can buoyant force be greater than the weight of an object?

Yes, buoyant force can be greater than the weight of an object. When this occurs, the object will float on the surface of the fluid. If the buoyant force is less than the object's weight, it will sink.

7. Why do some objects float while others sink?

Objects float or sink based on their average density compared to the fluid's density. If an object's average density is less than the fluid's density, it will float. If it's greater, the object will sink. This explains why a steel ship can float while a small steel ball sinks.

8. How does buoyant force relate to Archimedes' principle?

Archimedes' principle states that the buoyant force acting on an object immersed in a fluid is equal to the weight of the fluid displaced by the object. This principle is the fundamental basis for understanding buoyant force and its effects.

9. How does the shape of an object affect its buoyancy?

The shape of an object affects its buoyancy by determining the volume of fluid it displaces. Objects with shapes that displace more fluid relative to their weight (like boats) are more buoyant than compact shapes of the same material and weight.

10. What is meant by "apparent weight" in the context of buoyancy?

Apparent weight is the perceived weight of an object when it is submerged in a fluid. It is less than the object's actual weight in air due to the buoyant force acting upward on the object, making it feel lighter.

11. What factors affect the magnitude of buoyant force?

The magnitude of buoyant force depends on three main factors: the density of the fluid, the volume of fluid displaced by the object, and the acceleration due to gravity. Increasing any of these factors will increase the buoyant force.

12. What is the formula for calculating buoyant force?

The formula for buoyant force is F_b = ρ * g * V, where F_b is the buoyant force, ρ (rho) is the density of the fluid, g is the acceleration due to gravity, and V is the volume of fluid displaced by the object.

13. What is the relationship between buoyant force and fluid pressure?

Buoyant force is a result of the difference in fluid pressure between the top and bottom of an immersed object. The pressure increases with depth, so the upward force on the bottom of the object is greater than the downward force on its top, resulting in a net upward buoyant force.

14. How does buoyant force relate to the concept of displacement in fluids?

Buoyant force is directly related to displacement. The force is equal to the weight of the fluid displaced by the immersed object. This means that the more fluid an object displaces, the greater the buoyant force acting on it.

15. How does changing the depth of an object in a fluid affect the buoyant force on it?

The buoyant force on a fully submerged object remains constant regardless of its depth. This is because the volume of fluid displaced (which determines the buoyant force) doesn't change with depth for a fully submerged object of fixed volume.

16. How does buoyant force explain why it's easier to float in saltwater than in freshwater?

Saltwater has a higher density than freshwater. According to the buoyant force formula, a higher fluid density results in a greater buoyant force. This increased force makes it easier for objects (including humans) to float in saltwater compared to freshwater.

17. How does buoyant force affect the apparent weight of an object in different fluids?

The apparent weight of an object in a fluid is its true weight minus the buoyant force. In denser fluids, the buoyant force is greater, so the apparent weight is less. This is why objects feel lighter in water than in air, and even lighter in denser fluids like mercury.

18. How do fish control their buoyancy in water?

Fish control their buoyancy using a swim bladder, an internal organ that can be filled with or emptied of gas. By adjusting the amount of gas in the swim bladder, fish can change their overall density, allowing them to achieve neutral buoyancy at different depths.

19. How does the concept of buoyant force apply to hot air balloons?

Hot air balloons work by heating the air inside the balloon, which decreases its density. This creates a buoyant force as the less dense hot air inside the balloon is surrounded by cooler, denser air outside. When this buoyant force exceeds the total weight of the balloon and its cargo, the balloon rises.

20. What role does buoyant force play in the design of submarines?

Submarines use buoyant force principles to control their depth. They have ballast tanks that can be filled with water or air. Filling the tanks with water increases the submarine's density, causing it to sink, while filling them with air decreases density, allowing it to rise.

21. What is the difference between positive, neutral, and negative buoyancy?

Positive buoyancy occurs when an object's density is less than the fluid's, causing it to float. Neutral buoyancy is when the object's density equals the fluid's, allowing it to remain suspended. Negative buoyancy happens when the object's density is greater than the fluid's, causing it to sink.

22. What is the concept of "center of buoyancy" and how does it differ from the center of gravity?

The center of buoyancy is the centroid of the displaced fluid volume, while the center of gravity is the average location of the weight of an object. For a floating object to be stable, the center of buoyancy must be directly above the center of gravity. Their relative positions determine the object's stability in the fluid.

23. What is the role of buoyant force in the functioning of a hydrometer?

A hydrometer uses the principle of buoyancy to measure the density of liquids. It floats at different levels in liquids of different densities. In denser liquids, it experiences a greater buoyant force and floats higher. By calibrating the stem, the hydrometer can directly indicate the liquid's density or specific gravity.

24. How does the shape of a ship's hull affect its buoyancy and stability?

A ship's hull shape affects both its buoyancy and stability. A wider, flatter bottom increases the volume of water displaced, providing greater buoyancy. The hull shape also influences the location of the center of buoyancy relative to the center of gravity, which determines the ship's stability. A lower center of gravity and a higher center of buoyancy generally increase stability.

25. What is the concept of "reserve buoyancy" in naval architecture?

Reserve buoyancy is the volume of the watertight portion of a ship's hull above the waterline. It represents the additional volume that can be submerged before the ship sinks. More reserve buoyancy means the ship can take on more water or cargo without sinking, increasing its safety and versatility.

26. What is buoyant force?

Buoyant force is the upward force exerted by a fluid on an object that is partially or fully immersed in it. This force opposes the weight of the immersed object and is responsible for making objects float or appear lighter in fluids.

27. Can buoyant force be negative?

Buoyant force is always positive (upward) in a gravitational field. What can be negative is the net force on an object, which is the sum of its weight (downward) and the buoyant force (upward). If the weight exceeds the buoyant force, the net force is downward, causing the object to sink.

28. Can buoyant force exist in gases as well as liquids?

Yes, buoyant force exists in all fluids, including gases. This is why helium balloons rise in air and how hot air balloons work. The principle is the same as in liquids, but the effects are often less noticeable due to the lower density of gases.

29. Can an object experience zero buoyant force? If so, under what conditions?

An object can experience zero buoyant force only when it is completely outside a fluid (in a vacuum or air for most practical purposes). As soon as any part of the object enters a fluid, it will experience some degree of buoyant force.

30. How does the principle of buoyancy apply to icebergs floating in the ocean?

Icebergs float because ice is less dense than seawater. The principle of buoyancy dictates that the weight of the water displaced by the submerged part of the iceberg equals the weight of the entire iceberg. This is why about 90% of an iceberg's volume is below the water surface.

31. What is the relationship between buoyant force and Pascal's principle?

While not directly related, both principles are fundamental to fluid mechanics. Pascal's principle states that pressure applied to a confined fluid is transmitted equally in all directions. This uniform pressure distribution contributes to the buoyant force by creating a greater upward force on the bottom of a submerged object than the downward force on its top.

32. How does atmospheric pressure affect buoyant force in liquids?

Atmospheric pressure doesn't directly affect the buoyant force in liquids. The buoyant force depends on the difference in pressure between the top and bottom of the immersed object, which is determined by the fluid's density and the object's depth, not the atmospheric pressure above the liquid.

33. How does buoyant force affect the measurement of an object's mass using a spring scale in different fluids?

A spring scale measures the apparent weight, which is affected by buoyant force. In denser fluids, the buoyant force is greater, making the object appear lighter. This means the same object will show different weights on a spring scale when measured in air, water, or oil, despite its mass remaining constant.

34. What is the role of buoyancy in the formation of cumulus clouds?

Cumulus clouds form due to buoyancy in the atmosphere. As the sun heats the Earth's surface, warm air near the ground becomes less dense and rises due to buoyant force. As this air parcel rises, it cools and its water vapor condenses, forming the characteristic puffy shape of cumulus clouds.

35. How does the concept of buoyant force apply to the layers of the Earth's atmosphere?

In the atmosphere, less dense warm air rises due to buoyant force, while denser cold air sinks. This principle drives atmospheric convection, which is crucial for weather patterns. It's similar to how less dense liquids float on denser ones, but occurs continuously in the atmosphere due to temperature variations.

36. What is metacentric height, and how does it relate to buoyancy?

Metacentric height is a measure of the initial static stability of a floating body. It's the distance between the center of gravity and the metacenter (the point around which the body oscillates). A larger metacentric height indicates greater stability. While not directly related to buoyant force, it's crucial for understanding how floating objects behave when disturbed.

37. What is the relationship between buoyant force and the volume of an object?

The buoyant force is directly proportional to the volume of fluid displaced by the object, not the object's volume itself. For a fully submerged object, this is equal to the object's volume. For partially submerged objects, it's the volume of the part below the fluid surface.

38. How does the salinity of water affect buoyant force?

Increasing the salinity of water increases its density. According to the buoyant force formula (F_b = ρgV), a higher fluid density (ρ) results in a greater buoyant force. This is why it's easier to float in the Dead Sea, which has very high salinity, compared to a freshwater lake.

39. How does the concept of buoyancy apply to the design of life jackets?

Life jackets are designed to provide additional buoyancy to a person in water. They contain materials that are less dense than water, increasing the volume of water displaced without significantly increasing weight. This additional displaced water volume increases the buoyant force, helping to keep the person afloat.

40. What is Archimedes' paradox, and how does it relate to buoyant force?

Archimedes' paradox refers to the counterintuitive fact that a larger mass of fluid can be held up by a smaller mass of the same fluid. This is explained by buoyant force: the upward force on the bottom of a container can balance the weight of a much taller column of fluid above it, as long as the pressures are equal at the same depth.

41. How does buoyant force contribute to convection currents in fluids?

Buoyant force drives convection currents in fluids. When a portion of fluid is heated, it expands and becomes less dense. The buoyant force then causes this less dense fluid to rise. As it rises and cools, denser fluid sinks to take its place, creating a continuous convection current. This principle is crucial in understanding weather patterns, ocean currents, and heat transfer in fluids.

42. How does the principle of buoyancy apply to the design of hovercrafts?

Hovercrafts use a combination of buoyancy and air pressure to float just above a surface. They create a cushion of high-pressure air underneath, which provides a buoyant force. This force, combined with the lift from the downward-directed fans, allows the hovercraft to float on a thin layer of air, reducing friction with the surface below.

43. What is the concept of "meta-stable equilibrium" in relation to floating objects?

Meta-stable equilibrium refers to a state where a floating object is balanced but can easily be tipped over. This occurs when the metacenter is only slightly above the center of gravity. While the object is technically stable, small disturbances can cause it to capsize. Understanding this concept is crucial in ship design to ensure adequate stability.

44. How does the presence of dissolved gases in a liquid affect its buoyant force?

Dissolved gases generally decrease the density of a liquid slightly, which in turn decreases the buoyant force it exerts. However, this effect is usually minimal unless there's a significant amount of dissolved gas. In some cases, like carbonated beverages, the effect can be more noticeable, affecting the behavior of objects immersed in the liquid.

45. What is the relationship between buoyant force and the concept of specific gravity?

Specific gravity is the ratio of a substance's density to the density of a reference substance (usually water). It's directly related to buoyancy because objects with a specific gravity less than 1 will float in water, while those with a specific gravity greater than 1 will sink. The greater the difference between an object's specific gravity and that of the fluid, the stronger the buoyant effect.

46. How does the principle of buoyancy apply to the functioning of hot air balloons at different altitudes?

As a hot air balloon rises, the air outside becomes less dense. To maintain altitude, the air inside the balloon must be heated further to maintain the density difference and thus the buoyant force. At higher altitudes, more heating is required to achieve the same buoyant force, which is why balloons have a maximum attainable altitude.

47. What is the concept of "neutral buoyancy" and how is it used in scuba diving?

Neutral buoyancy is the state where an object's weight is exactly balanced by the buoyant force, causing it to neither sink nor float. Scuba divers aim for neutral buoyancy to conserve energy and maintain depth easily. They achieve this by adjusting their buoyancy compensator device, which can add or remove air to change their overall buoyancy.

48. How does the principle of buoyancy contribute to the phenomenon of stratification in lakes and oceans?

Stratification in bodies of water occurs when layers of water with different densities form. Colder, denser water sinks to the bottom due to negative buoyancy, while warmer, less dense water rises to the top due to positive buoyancy. This creates distinct layers with minimal mixing, affecting nutrient distribution and aquatic ecosystems.

49. What is the role of buoyant force in the design and function of submarine ballast systems?

Submarine ballast systems control the vessel's buoyancy by adjusting the amount of water in ballast tanks. To dive, the submarine takes on water, increasing its density and reducing buoyancy. To surface, compressed air is used to expel water from the tanks, decreasing density and increasing buoyancy. This allows precise control of depth and attitude underwater.

50. How does the concept of buoyancy apply to the phenomenon of magma rising through the Earth's crust?

Magma rises through the Earth's crust due to buoyancy. As magma is generally less dense than the surrounding solid rock, it experiences an upward buoyant force. This force, combined with pressure from below, causes magma to rise through cracks and weaknesses in the crust, potentially leading to volcanic eruptions.

51. What is the relationship between buoyant force and the concept of hydrostatic equilibrium in stars?

In stars, hydrostatic equilibrium is the balance between the