Have you ever experienced feeling lighter when swimming in the pool or a bucket feeling lighter when drawing water from a well? If you do, buoyancy occurs during those events. In buoyancy, when immersed in water or any source of fluid, a body experiences a force from the downward direction opposite to the direction of the gravitational pull.
This is why we experience a decrease in weight when such happens. Also, this may be the reason why needles sink and plastic bottles float regardless of the weight.
Well, in this reading, we’ll explore what buoyancy is, its diagram, applications, purpose, causes, types, and how buoyancy is attained. You’ll also get to know the density and related density and why do object float or sink in water
Let’s begin!
Learn How Force Changes The State Of Motion with this detailed guide!
What is Buoyancy?
Buoyancy, or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. It is also said to be the force that causes objects to float. Buoyancy occurs when there are differences in pressure acting on opposite sides of an object immersed in a static fluid.
It is also called buoyant force because buoyancy is the phenomenon due to buoyant force. In a column of fluid, pressure increases with depth due to the weight of the overlying fluid. Therefore, the pressure at the bottom of a column of fluid is greater than that at the top of the column.
Similarly, the pressure at the bottom of an object submerged in a fluid is greater than that at the top of the object. This pressure difference results in a net upward force on the object.
Archimedes’ principle also explained that the magnitude of the force is proportional to the pressure difference and is equivalent to the weight of the fluid that would otherwise displace the liquid; the force can keep the object afloat. This is why an object whose average density is greater than that of the fluid in which it is submerged tends to sink.
But if the object is less dense than the fluid, the force can keep the object afloat. This will only occur in a non-inertial reference frame that has a gravitational field or is accelerating due to a force other than gravity. This is defining a “downward” direction.
The unit of a buoyant force is the Newton (N)
So, buoyancy can be defined as the upward force applied by the fluid on the object or the body when an object is put in or submerged in the fluid.
The center of buoyancy of an object is the center of gravity of the displaced volume of fluid. It is the point on the object where the force acts or the point where the force of buoyancy is applied. The force of buoyancy is vertical, so the center of buoyancy is the point situated on the center of the gravity of the liquid that is being displaced by the object submerged.
You should also learn about fluid mechanics with this detailed guide!
Applications
The application of buoyancy is so vast since it can keep objects afloat in water, something we humans cannot do without. Buoyancy is used in submarines and hot air balloons; even fish and swimmers use the same principle. Below are some applications of buoyancy in the real world.
- Hot air balloon: Because the atmosphere is containing air that exerts a buoyant force on any object. A balloon with hot air will float due to the buoyant force, it will descend when the balloon’s weight is higher than the buoyant force. This is because the balloon becomes stationary when the weight equals the buoyant force.
- Submarine: Submarines make good use of the principle of buoyancy. A submarine has a large ballast tank that is used to control its position and depth from the surface of the sea. In submarines, water is allowed to enter into the ballast tank so that its weight becomes greater than the buoyant force.
- Ship: Ships can float on the surface of the sea due to the volume of water displaced by the ship is enough to have a weight equal to the weight of the ship. Ships are constructed to have hollow shapes, which make the overall density of a ship less than the seawater. This is why the buoyant force acting on the ship is large enough to support its weight.
- Fish: Just as earlier stated, some certain groups of fishes and swimmers also use the principle of Archimedes’ to move up and down in the water. To go up to the water surface, the fishes will fill their swim bladder (air sacs) with gases. These gases diffuse from their own body to the bladder, making their body lighter, which enables the fish to go up.
Formula
The formula for buoyancy (also known as Archimedes’ Principle) is
Buoyant Force (Fb)=ρ⋅V⋅g\text{Buoyant Force (} F_b \text{)} = \rho \cdot V \cdot g
Where:
-
FbF_b = Buoyant Force (in newtons, N)
-
ρ\rho = Density of the fluid (in kg/m³)
-
VV = Volume of the fluid displaced (in m³)
-
gg = Acceleration due to gravity (≈ 9.81 m/s²)
Example:
If an object displaces 0.5 m³ of water (density = 1000 kg/m³), the buoyant force is:
Fb=1000⋅0.5⋅9.81=4905 NF_b = 1000 \cdot 0.5 \cdot 9.81 = 4905 \, \text{N}
This means the water exerts an upward force of 4905 newtons on the object.
Learn about constrained motion with this detailed guide!
Causes and Examples of Buoyancy
It is mandatory to know that buoyancy is caused by the pressure exerted by the fluid in which the object is immersed. Also, the buoyant force experienced by the object is always upwards. This is because the pressure of the fluid increases with the depth. Some examples of buoyancy also include:
- A boat or a ship floating in the water
- An immersed cork in water will end up floating due to buoyancy.
- Swimmers are good examples of buoyancy.
- A boat floating on water – The weight of the boat is balanced by the upward buoyant force from the water.
- A hot air balloon rising in the sky – Warm air inside the balloon is less dense than cooler air outside, creating buoyancy.
- Iceberg floating in the ocean – Ice is less dense than seawater, so it floats.
- A swimmer floating on their back – Human body can float because of air in the lungs and body fat, which is less dense than water.
- A piece of wood floating in a river – Wood has a lower density than water, so it floats.
- A rubber duck floating in a bathtub – The duck is buoyant due to its hollow, lightweight design.
- A life jacket keeping a person afloat – Life jackets trap air and increase overall buoyancy.
- An oil spill floating on seawater – Oil is less dense than water, causing it to float.
- A helium balloon rising in the air – Helium is lighter than air, causing the balloon to be buoyant.
- A fish adjusting its depth using a swim bladder – The fish controls buoyancy by changing the gas volume in its swim bladder.
- A submarine floating or diving by adjusting ballast tanks – Submarines regulate buoyancy using air and water in their tanks.
- A beach ball bobbing on ocean waves – It’s filled with air and displaces enough water to stay afloat.
- A cargo ship floating while carrying heavy containers – Despite the load, the hull displaces enough water to remain buoyant.
- A sealed empty bottle floating in a lake – The air inside keeps it buoyant.
- A lava lamp’s blobs rising and falling – Buoyancy changes due to heating and cooling of the wax blobs.
- A paper boat floating in a puddle – The lightweight paper structure displaces enough water to stay afloat (temporarily).
- Floating lanterns during festivals – Hot air inside makes them buoyant in the cooler surrounding air.
- A plastic container floating when tossed into a pool – The container is less dense than water.
- Air bubbles rising to the surface of a soda – Bubbles are lighter than the liquid, so buoyancy pushes them up.
- A tire tube floating in a river – Filled with air, it displaces enough water to support weight and float.
Types of Buoyancy
Below are the various types of buoyancy. The three types of buoyancy are positive, negative, and neutral buoyancy.
Positive types of buoyancy occur when the immersed object is lighter than the fluid displaced. This is why the object floats. Negative buoyancy types are when the immersed object is denser than the fluid displaced, resulting in the object sinking.
Finally, neutral types of buoyancy occur when the weight of the immersed object is equal to the fluid displaced. Below are the factors that can affect the various types of buoyancy.
- The density of the fluid.
- The volume of the fluid displaced can also affect buoyancy. Lastly,
- The local acceleration due to gravity.
Factors that do not affect buoyant force include, the density of the immersed object and the mass of the immersed object is also a factor.
Density and relative density
To understand the concept of buoyancy, one must properly understand the concept of density and relative density.
The density of materials can be defined as their mass per unit volume. It is a measurement of how tightly matter is packed together. It is numerically defined as:
Density, ρ=MassVolume=MV
- The SI unit of density is measured using kilograms per cubic meter (kg/m3).
- The density is 0.9584 grams per cubic centimetre at 100° Celsius.
The relative density of a substance is defined as the ratio of the density of the substance to the density of water at C. It is also known as the specific gravity of a substance.
Relative Density = Density of a substance/Density of water at 4°C
Since relative density is a ratio of similar quantities, it has no unit.
You can also learn about fluid with this detailed guide!
Why do object float or sink in water
The reason objects float or sink in water can be considered when one imagines water consists of overlying layers. That is one over the other with varying pressure. The pressure at the bottom of the liquid is greater than that at the top, which is why we sink in the liquid in some layers. These layers tend to increase as the object sinks.
With the difference in pressure in the fluid layers, a made-up force is being applied in its upward direction. This force results in the acceleration of the object that has been submerged in the upward direction. The force is always in the vertical direction.
In other words, it can also be said that the magnitude of the upward force is basically equivalent to the difference in the pressure of the topmost. The last layer is also equivalent to the weight of the fluid that has been displaced.
The consequence of the above concept is what we call floating. The object should be less dense than the water; otherwise, greater density will result in the sinking of the object.
Conclusion
Buoyancy is the upward force exerted by a fluid that opposes the weight of an object submerged in it. This fundamental principle explains why objects float or sink and plays a crucial role in fluid mechanics, shipbuilding, aeronautics, and even weather systems.
Governed by Archimedes’ Principle, buoyancy depends on the displaced fluid’s weight and the object’s density. Understanding buoyancy helps in designing floating structures, predicting fluid behavior, and solving real-world engineering and scientific problems.
FAQs on Buoyancy
What is buoyancy?
Buoyancy is the upward force exerted by a fluid (liquid or gas) that supports the weight of an object submerged in it.
What determines whether an object floats or sinks?
If the object’s density is less than the fluid’s density, it floats; if more, it sinks.
What is Archimedes’ Principle?
It states that the buoyant force on a submerged object is equal to the weight of the fluid it displaces.
Does buoyancy apply in gases too?
Yes, buoyancy applies in both liquids and gases—for example, how hot air balloons rise in the atmosphere.
Can buoyancy be negative?
Yes. When the weight of the object exceeds the buoyant force, it has negative buoyancy and sinks.
Why do ships made of steel float?
Although steel is dense, ships are designed with hollow shapes that displace enough water to create a buoyant force greater than their weight.
https://studentlesson.com/buoyancy-definition-applications-purpose-causes-types/