Fluid

Some things can flow. These are called fluids.

Liquids are fluids. Gases are fluids too. They can move and change shape. They fit inside a cup or a bowl.

Some fluids act like solids. Think of Silly Putty. It can act hard if you hit it fast.

Other fluids are very thick. They move very slowly. This is like thick black pitch.

Water is a fluid that stays the same size. Air can change size too. It is fun to study how things flow.

What is a fluid? A fluid is a material that can flow. This includes liquids and gases. Fluids change shape when you push them. They do not resist this change. This is different from solids. Solids can push back. They can even snap or break.

Some fluids are special. We call these non-Newtonian fluids. These act like solids if you hit them fast. Silly Putty is one example. Other fluids are very thick. Pitch is a fluid that moves very slowly. It can look like a solid.

We can group fluids in different ways. Some fluids are compressible. This means they change size when you press them. Other fluids are incompressible. This means their size stays the same. Water and oil are examples of this. Scientists often use these ideas to make models.

People who study fluids use fluid mechanics. This field has two parts. Fluid dynamics studies fluids that move. Fluid statics studies fluids that stay still. It is a big way to learn about our world.

A fluid is a material that can flow. This includes both liquids and gases. Fluids change shape when an external force is applied to them. They have something called zero shear modulus. This means they cannot resist a force that tries to make them slide. Because they flow, they take the shape of their container. This is very different from a solid. Solids can push back against a force. They might even snap or break if you push too hard.

How a fluid works depends on how it reacts to pressure. Fluids respond to normal stresses, which we call pressure. They can handle compressive stress, which is positive pressure. They can also handle tensile stress, which is negative pressure. Some fluids are also classified by how they change size. A compressible fluid changes its volume when you apply pressure. An incompressible fluid does not change its volume. Water and oil are examples of incompressible fluids.

Scientists use different names to group these materials. Some are called Newtonian fluids. These follow Newton's law of viscosity. In these fluids, the stress is proportional to the rate of strain. Other materials are called non-Newtonian fluids. In these, the stress is not proportional to the rate of strain. Silly Putty is a famous example of this. It acts like a solid when you apply a sudden force. Pitch is another example because it is very thick.

There are many ways to study these moving parts. The study of fluids is called fluid mechanics. This field has two main branches. One branch is fluid dynamics, which looks at fluids in motion. The other branch is fluid statics, which looks at fluids that stay still. Scientists use the Navier–Stokes equations to model them. These equations are based on mass, energy, and momentum. They help us understand how things like air and water move.

You can see fluids working in many places every day. In medicine, doctors talk about body fluids. This refers to the liquids inside a living body. When someone is sick, they might need to drink more fluids. In machines, hydraulics use liquids with special properties to work. Even the air around you is a gas, which is a fluid. Understanding fluids helps us understand everything from tiny droplets to huge oceans.

In the world of physics, a fluid is a substance that can flow. This category includes both liquids and gases. A fluid is defined by its ability to move and deform under an applied shear stress. Shear stress is an external force that tries to make parts of a substance slide past each other. Because fluids have a zero shear modulus, they cannot resist these sliding forces. This means they will continuously change shape when such a force is applied.

To understand how a fluid works, we must look at how it reacts to different types of stress. Solids respond to shear stress with a spring-like restoring force. This means a solid can often return to its original shape after being pushed. Fluids behave very differently. Instead of resisting the force itself, they only resist the rate at which they deform. This resistance is often described in a frictional manner.

Fluids also react to normal stresses, which we call pressure. A fluid can handle compressive stress, which is known as positive pressure. It can also handle tensile stress, which is known as negative pressure. This is a key difference from solids. While both solids and liquids have tensile strength, exceeding it causes different results. In solids, it leads to permanent deformation or fractures. In liquids, it causes a phenomenon called cavitation.

Scientists classify fluids into different types based on their specific behaviors. One way to group them is by their relationship between shear stress and the rate of strain. Newtonian fluids follow Newton's law of viscosity. In these fluids, the stress is directly proportional to the rate of strain. However, non-Newtonian fluids do not follow this rule. In a non-Newtonian fluid, the stress is not proportional to the rate of strain or its derivatives.

Another way to classify fluids is by their compressibility. A compressible fluid is one that changes its volume or density when pressure is applied. This can also happen when the fluid becomes supersonic. In contrast, an incompressible fluid does not change its volume when pressure or flow velocity changes. Water and oil are examples of incompressible fluids. It is important to note that in the real world, perfectly Newtonian or incompressible fluids do not actually exist. Scientists use these terms as theoretical assumptions to help solve complex problems.

Some substances can even appear to be both solids and fluids. For example, Silly Putty is a non-Newtonian fluid that acts like a solid when a sudden force hits it. Pitch is another substance with extremely high viscosity. Because it is so thick, it can behave like a solid, as seen in the pitch drop experiment. Even in particle physics, the concept of a fluid is extended to include matters other than just liquids and gases.

The study of these materials is called fluid mechanics. This field is divided into two main branches. Fluid statics looks at fluids that are not moving. Fluid dynamics focuses on fluids that are in motion. To model these complex movements, scientists use the Navier–Stokes equations. These are a set of partial differential equations. They are based on the conservation of mass, energy, and both linear and angular momentum.

Understanding fluids is vital across many different scientific fields. In medicine and biology, the term "fluid" refers to the liquid constituents of the body. When doctors suggest drinking fluids, they are talking about liquid replacement. In the field of hydraulics, the term refers to liquids with specific properties used to move things. From the air we breathe to the blood in our veins, fluids are essential to how the world works.