The pressure and density of fluids

The pressure and density of fluids
The pressure and density of fluids

In The pressure and density of fluids lesson, you will understand how to define the main properties that characterize fluids in their state of rest. , you will also learn how they are related and how fluids with different characteristics interact with each other.

In the context of fluid mechanics, pressure and density are two fundamental concepts that play a key role in determining the behavior of fluids. Whether liquid or gas, these two parameters are essential to understanding how fluids move, interact with solid surfaces and respond to changes in temperature and pressure. Let’s discover them together in this article!

Fluids: what they are

Even in everyday life, we ​​deal with fluids and their properties. When we open a bottle of sparkling water and notice the gas coming out of the cap, we have actually opened a container that was under pressure , forcing the pressure to lower to atmospheric pressure.

When we brush our teeth in the morning and press on the toothpaste tube we are actually increasing the pressure of the fluid contained inside it so as to force it out of the package. When we combine multiple liquid ingredients together for a recipe and don’t see them mix until we mix, we are actually observing the effect of different densities of fluids placed in the same container. There can be many practical examples and they are all closely linked to the concept of pressure and density of fluids.

The term fluids refers to any substance that is in a liquid or gaseous state. In general, in fact, liquids and gases have very similar properties and behave in a similar way when subjected to states of stress. For this reason, they are generally distinguished from solids and united under the same category: fluids.

Among them, there is a sub-category that is simpler to study than the others and which is represented by those fluids called incompressible, for which the density never changes. In this chapter we will mainly deal with this group.

In physics, fluid analysis focuses on the laws of motion and interactions between fluids and solid surfaces. This study is known as fluid mechanics and has applications in a wide range of fields, including engineering, meteorology, medicine, and ecology.

The density of fluids: how can we define it

The pressure and density of fluids
The pressure and density of fluids


Density is a measure of the amount of mass contained in a given volume of a fluid . In other words, it describes how “heavy” a fluid is in a given space, and is usually measured in kilograms per cubic meter (kg/m³). Density plays a crucial role in determining how fluids flow and react to external forces, influencing phenomena such as buoyancy and laminar flow . Understanding density is critical in many applications, such as ship design, weather prediction, and ocean current analysis.

The density of a substance generally expresses the ratio between its mass and its volume . It can therefore be described by the following relation:


In other words, density is an indicator of how much more a substance “weighs” than another, for the same occupied volume. Different fluids will have different densities and will therefore interact with each other depending on this difference. For example, try a classic experiment : fill half a glass of water and then pour in a drizzle of oil. The oil will settle in the upper part of the glass, remaining separated from the water below. This happens because the density of the oil is lower than that of water, consequently the former will not sink into the latter, but will instead arrange itself to form a layer in the upper part of the glass, being lighter.

Fluid pressure: what is it

Pressure is described as a force exerted on a surface. The pressure of a fluid can be expressed in different units of measurement , as we will see later. It can also refer to vacuum ( absolute pressure ) or atmospheric pressure ( relative pressure ).

Pressure is a measure of the force exerted by a fluid on a surface per unit area. It can be thought of as the intensity of the “knock” that the fluid gives on a surface , and is often measured in Pascals (Pa) or atmospheres (atm). In a fluid, pressure can vary with depth, location, and other factors, and understanding it is vital to designing systems such as plumbing, aircraft, and underwater equipment.

As with density , the concept of pressure of a fluid is actually very familiar to us and we have the opportunity to observe its implications every day. Think for example of a fountain: the water is able to gush higher depending on the pressure supplied to it.

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