Buoyancy. Archimedes’ Principle and density

23/04/2026

The online buoyancy simulations on this page will help you understand the physical fundamentals that make some objects float and others not. We will discover Archimedes’ Principle and we will see what is the relationship between buoyancy and density.

This Thematic Unit is part of our Physics collection

Apparent Weight

Net force acting on a submerged body, resulting from the difference between its real weight and the buoyant force (Pa = P – E) in the SI.

Archimedes’ Principle

Law stating that any body immersed in a fluid experiences an upward buoyant force equal to the weight of the displaced fluid.

Buoyant Force

Upward vertical force exerted by a fluid on a submerged body, calculated as E = ρ * V * g in the SI (in Newtons).

Center of Buoyancy

Point where the buoyant force is considered to be applied, coinciding with the center of gravity of the displaced fluid volume.

Density

Scalar quantity expressing the amount of mass per unit volume of a substance, measured in kg/m³ in the SI.

Displaced Volume

Space occupied by the fluid that has been moved aside by the body upon immersion, measured in m³ in the SI.

What is buoyancy

Buoyancy is the ability of bodies to stay on the surface of a liquid or in the air without sinking. It is a fundamental physical principle governed by Archimedes’ Principle, enunciated by the Greek scientist of the same name.

Archimedes’ principle. Buoyancy force

Archimedes’ principle is a physical law established by the Greek scientist Archimedes, which describes the behavior of bodies immersed in fluids, either liquids or gases. This principle states that a body fully or partially immersed in a fluid will experience a vertical and upward thrust equal to the weight of the fluid displaced by the body. When a body is immersed in a fluid, either completely or partially, the fluid exerts a pressure on the body in all directions. The thrust is the resultant force of all these pressures and is equal to the weight of the fluid displaced by the body. If the weight of the object is less than the thrust, the body will float. If the weight is greater, the body will sink.

Relationship between buoyancy and density

Density is a physical property that relates the mass of an object to its volume, in the SI it is expressed in kilograms per cubic meter (kg/m³). It is a key factor in understanding buoyancy, as it determines whether an object sinks or floats when immersed in a fluid. This relationship is fundamental in the study of Archimedes’ principle and therefore, knowing the density of materials allows us to predict their behavior in water or other fluids.

Applications of the buoyancy principle

The principle of buoyancy is applied in many areas, such as ship design and construction. In the case of ships, for example, the shape and volume of the hull allows enough water to be displaced so that the total weight of the ship is less than the thrust generated by the water, thus ensuring its buoyancy.

The online buoyancy simulations on this page are an excellent tool to learn more about this important physics concept, to know Archimedes’ principle and to understand the relationship between buoyancy and density. Don’t miss them!

Apparent Weight

Net force acting on a submerged body, resulting from the difference between its real weight and the buoyant force (Pa = P – E) in the SI.

Archimedes’ Principle

Law stating that any body immersed in a fluid experiences an upward buoyant force equal to the weight of the displaced fluid.

Buoyant Force

Upward vertical force exerted by a fluid on a submerged body, calculated as E = ρ * V * g in the SI (in Newtons).

Center of Buoyancy

Point where the buoyant force is considered to be applied, coinciding with the center of gravity of the displaced fluid volume.

Density

Scalar quantity expressing the amount of mass per unit volume of a substance, measured in kg/m³ in the SI.

Displaced Volume

Space occupied by the fluid that has been moved aside by the body upon immersion, measured in m³ in the SI.

Explore the exciting STEM world with our free, online, simulations and accompanying companion courses! With them you’ll be able to experience and learn hands-on. Take this opportunity to immerse yourself in virtual experiences while advancing your education – awaken your scientific curiosity and discover all that the STEM world has to offer!

Buoyancy simulations

Buoyancy and density

This simulation allows us to understand the relationship between buoyancy and density. See how the density of an object changes as its mass or volume changes and how this affects its buoyancy.

Buoyancy I

With this simulation, you can easily explore the fundamental principles of flotation. Adjust the mass, volume, and material type of a block and observe how its behavior changes in different fluids. Check under what conditions an object sinks or floats, and see how relative density determines equilibrium, putting Archimedes’ principle into practice interactively.

Buoyancy II

This simulation takes the study of flotation a step further. Experiment with objects of different shapes and materials, change the fluid, and observe how the forces involved act: gravity, buoyancy, and contact. Explore more realistic scenarios, such as bottles and ships, to understand how Archimedes’ principle applies in everyday life and in the design of floating objects.

Archimedes’ principle

This simulation of Archimedes’ principle allows us to study the fundamental concepts of Archimedes’ principle. Change the mass and volume of the object and see what happens.

Buoyancy lab

Balance

Immerse the two objects in water. Why are they unbalanced? How do you achieve the point of equilibrium? Can you explain it?

Giants of science

“If I have seen further, it is by standing on the shoulders of giants”

Isaac Newton

Evangelista Torricelli

1608

–

1647

Evangelista Torricelli invented the mercury barometer, demonstrating the existence of atmospheric pressure, and made advances in hydrodynamics, geometry, and applied optics

“We have only today; let us make good use of this day”

Blaise Pascal

1623

–

1662

Blaise Pascal studied fluid pressure, mathematical statistics, and laid foundations of probability theory

“The understanding of pressure and vacuum transforms our comprehension of nature”

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Joseph Louis Gay-Lussac

1778

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1850

Joseph Louis Gay-Lussac formulated fundamental gas laws on the relationship between volume, temperature, and pressure, also studying gas mixtures and chemical reactions in air

“Chemistry is the key to understanding nature”

Archimedes

287 a.C.

–

212 a.C.

Archimedes established fundamental principles of mechanics and hydrostatics, including the famous Archimedes’ principle of buoyancy in fluids

“Give me a place to stand, and I will move the world”

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Test your knowledge

What is buoyancy, and how does Archimedes’ Principle explain the behavior of objects in a fluid?

Buoyancy is the ability of an object to remain afloat or rise within a fluid. It is explained by Archimedes’ Principle, which states that any object fully or partially submerged in a fluid experiences an upward force equal to the weight of the fluid it displaces. This force arises because fluid pressure increases with depth, so the bottom of the object receives a stronger push than the top. The difference between these pressures creates the upward buoyant force. If this force is greater than the object’s weight, the object floats; if it is smaller, it sinks. Buoyancy is essential for understanding natural phenomena and technological applications such as ship design, submarine operation and density‑based measurement devices.

How are density, buoyant force and the behavior of submerged objects related?

Density is crucial for predicting whether an object will float, sink or remain suspended in a fluid. According to Archimedes’ Principle, the buoyant force depends on the volume of displaced fluid. If an object is less dense than the fluid, it displaces enough fluid to generate a buoyant force greater than its weight, so it floats. If it is denser, the buoyant force is insufficient and it sinks. When densities match, the object stays in equilibrium. This relationship is fundamental in naval engineering, oceanography, meteorology and the design of instruments such as hydrometers. Understanding how density and buoyant force interact allows us to predict material behavior, optimize floating structures and analyze natural processes like iceberg stability.

Why do some objects float while others sink?

It depends on density. If an object is less dense than the water, it displaces enough water to receive a buoyant force greater than its weight, so it floats. If it is denser, the buoyant force is too small and it sinks. That is why wood floats and rocks sink. It is not about how “heavy” something feels, but about how much mass it has compared to its volume.

What does Archimedes’ Principle say in simple terms?

It says that when you put an object in a liquid, the liquid pushes it upward with a force equal to the weight of the water the object displaces. If that upward push is stronger than the object’s weight, it floats; if not, it sinks. It is like the water giving the object a lift from below.

Why is buoyancy important in everyday life?

Buoyancy is essential for designing ships, submarines, buoys and floating structures. It also explains why hot‑air balloons rise, why we float more easily in saltwater and how aquatic animals stay at certain depths. In engineering, meteorology and oceanography, buoyancy helps predict how fluids and objects interact. It is a simple idea with huge practical impact.