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Characteristics and Properties of Fluids in Physics

Katherine Kaylegian-Starkey, Elizabeth Friedl
  • Author
    Katherine Kaylegian-Starkey

    Katherine has a bachelor's degree in physics, and she is pursuing a master's degree in applied physics. She currently teaches struggling STEM students at Lane Community College.

  • Instructor
    Elizabeth Friedl

    Elizabeth, a Licensed Massage Therapist, has a Master's in Zoology from North Carolina State, one in GIS from Florida State University, and a Bachelor's in Biology from Eastern Michigan University. She has taught college level Physical Science and Biology.

Learn what a fluid is, and understand the properties and characteristics of fluids and in physics. Consider the difference between fluids and solids. Updated: 12/02/2021

Fluid Definition in Physics

Fluids are generally regarded as objects that flow and conform to fill their surroundings. In physics, the definition of a fluid is more specific. Here, fluids refer to any material that cannot withstand a tangential shearing force and is constantly changing shape in response to said shearing force.

Hydrodynamics is the study of the movement of fluids. When discussing fluids, it is firstly important to understand that the term fluid is an umbrella category that includes both liquids and gases.

Difference Between a Fluid and a Liquid

The terms fluid and liquid are often used interchangeably, but they are in fact distinct terms. A fluid refers to any material that does not resist a tangential shearing force. Water, mercury, and oxygen are all examples of fluids because these are materials that, when compressed, yield to the compressive force.

Consider for example, water in a glass. The water will conform to the shape of the glass, then immediately change shape when it is poured out of the glass. While water, mercury, and oxygen are all fluids, they are not all liquids: water and mercury are liquids, whereas oxygen is a gas.

A liquid is a type of fluid that has a definite volume but no definite shape. A liquid cannot expand indefinitely (like a gas), and it cannot be infinitely compressed because of the forces between the molecules in the liquid.

Solids are not considered fluids because they do not easily succumb to shearing forces. For example, when a handful of dice are poured into a glass, the dice will fill the glass, but there will remain open spaces between the dice because they retain their cubed shape and do not give in to the shearing force from the glass.

What Is a Fluid?

Let's take a trip to the beach. It's quite relaxing here, isn't it? The white sand, the blue water, the fresh air - just thinking about it makes me want to do some physics! Okay, that was a mean trick, but while we're here, we might as well use this great location and learn a little bit about the different phases of matter.

You are probably quite familiar with these already because they are solids (the sand), liquids (the water) and gases (the fresh air). Solids are pretty different from liquids and gases because they hold their shape. You can't put a square block into a round hole because the block is solid and won't conform to the shape of the hole.

Liquids and gases are more similar to each other because, unlike solids, they both conform to the shape of their container. They also both flow when moved around, and any material that flows is called a fluid. When you put air in your car tires, they stay inflated because the gas particles fill the entire space inside. Likewise, when you fill your bathtub, you can't keep the water on just one side because it flows to cover all the space of the tub.

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Physics of Fluids: Properties of Fluids

When studying fluids, there are five properties that govern hydrodynamics: compressibility, pressure, buoyancy, viscosity, and surface tension. Both liquids and gases obey the same laws of physics, but because liquids and gases have different characteristics, they can behave differently with respect to each property.

Compressibility

Compressibility is the change in the volume of a fluid in relation to a unit increase in pressure. Essentially, compressibility is a measure of how much a fluid will yield to a given pressure, and is a constant value that is dependent upon the material in question.

Pressure

In physics, pressure describes the amount of force (F) distributed over a particular area (A), and is written as {eq}\frac{F}{A} {/eq}. Fluids exert force on materials they are in contact with, and this can be expressed in terms of pressure. Fluid pressure is customarily expressed in terms of the density of the fluid, the gravitational constant, and the height below a fluid in which an object is submerged. This is mathematically written as:

{eq}P = \rho gh {/eq}, where {eq}\rho {/eq} stands for density, g stands for gravity, and h stands for height.

Figure 1 below shows how to derive this customary representation of pressure from the force (referred to here as weight, w) to area ratio.

Figure 1: Derivation of the customary representation of pressure in physics, P = pgh from the definition that pressure is the ratio of weight to area.

This is the derivation of the physics definition of pressure.

Buoyancy

Buoyancy describes the upward force on an object that is submerged within a fluid. The buoyant force in physics is described as {eq}F_b = -\rho gV {/eq}, where {eq}\rho {/eq} stands for density, g stands for gravity, and V stands for volume.

Buoyancy is a characteristic that naturally results from the fluid pressure. As an object submerges deeper into a fluid, there is greater pressure on the bottom of the object than there is at the top of the object. This creates a net upward force, which represents the buoyant force.

Viscosity

Viscosity is a fluid's ability to resist flow, and it arises from friction between the molecules of the fluid. Viscosity can be thought of as the "thickness" of a fluid. A fluid such as water, which has relatively low viscosity, flows readily from a tap. Contrast this to a thicker substance such as motor oil, ketchup, or honey, which will not flow as readily from a container. The difference in how easily these fluids flow reflects the differences in their viscosities. Put another way, highly viscous fluids flow less easily. Formally, viscosity is described as the ratio of the shearing stress on a fluid to the velocity of the fluid.

Figure 2: Ketchup is a highly viscous fluid.

Ketchup is a highly viscous fluid.

Properties of Fluids

There are certain properties that fluids share, though the specifics of these may be slightly different for each type of fluid. The main difference between the two fluids mentioned here is that gas particles are much farther apart than the particles of a liquid. Both will spread out to fill their container, but a liquid only does so beneath its surface.

This important difference helps us understand that a gas is compressible, which means its volume can easily be increased or decreased, while a liquid is incompressible, meaning its volume cannot easily be changed. In other words, you can more easily press gas particles together than you can the particles of a liquid. This is because there is more space between those gas particles, while the liquid particles are already about as close together as they can get.

One property that all fluids do share is that they have density. This is simply the amount of matter in a given space for that substance. Another way of saying this is that the density is the amount of matter per unit volume, or in equation form: density = mass/volume.

Pressure is another important characteristic of fluids. This is the force exerted over a given area. There are many different units that can be used to express pressure, like pounds per square inch (psi), millimeters of mercury (mmHg) and Newton per meter squared (N/m^2). It all depends on the force and the area you're measuring.

In a liquid, the pressure comes from the weight of the fluid and the weight of the air above it, which we call the atmosphere. You feel this pressure when you swim under water. The deeper you go, the more pressure you feel. This is because there is more weight pressing down on you from above as you increase your depth.

The same principle is true for atmospheric gases. The lower in elevation you are, the more weight and pressure you experience. But even though these gases are constantly putting pressure on you, you don't feel it because your body is the same pressure as the surrounding air. Fish experience the same thing underwater - their bodies are the same pressure as the surrounding fluid, so they don't notice the constant pressure the water exerts on them.

Buoyancy is another common characteristic of fluids. This is the upward force from a fluid, which is usually felt by some object in that fluid. You float in a pool because of buoyancy, and a hot air balloon rises into the sky - also because of buoyancy.

Buoyancy in a fluid comes from the pressure of the fluid itself. An object submerged in a fluid experiences pressure from all sides. But since the pressure from below is greater (because that part of the object is deeper), it creates an unbalanced force on the object and it's pushed upward, lifting it in the fluid and against the force of gravity.

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Video Transcript

What Is a Fluid?

Let's take a trip to the beach. It's quite relaxing here, isn't it? The white sand, the blue water, the fresh air - just thinking about it makes me want to do some physics! Okay, that was a mean trick, but while we're here, we might as well use this great location and learn a little bit about the different phases of matter.

You are probably quite familiar with these already because they are solids (the sand), liquids (the water) and gases (the fresh air). Solids are pretty different from liquids and gases because they hold their shape. You can't put a square block into a round hole because the block is solid and won't conform to the shape of the hole.

Liquids and gases are more similar to each other because, unlike solids, they both conform to the shape of their container. They also both flow when moved around, and any material that flows is called a fluid. When you put air in your car tires, they stay inflated because the gas particles fill the entire space inside. Likewise, when you fill your bathtub, you can't keep the water on just one side because it flows to cover all the space of the tub.

Properties of Fluids

There are certain properties that fluids share, though the specifics of these may be slightly different for each type of fluid. The main difference between the two fluids mentioned here is that gas particles are much farther apart than the particles of a liquid. Both will spread out to fill their container, but a liquid only does so beneath its surface.

This important difference helps us understand that a gas is compressible, which means its volume can easily be increased or decreased, while a liquid is incompressible, meaning its volume cannot easily be changed. In other words, you can more easily press gas particles together than you can the particles of a liquid. This is because there is more space between those gas particles, while the liquid particles are already about as close together as they can get.

One property that all fluids do share is that they have density. This is simply the amount of matter in a given space for that substance. Another way of saying this is that the density is the amount of matter per unit volume, or in equation form: density = mass/volume.

Pressure is another important characteristic of fluids. This is the force exerted over a given area. There are many different units that can be used to express pressure, like pounds per square inch (psi), millimeters of mercury (mmHg) and Newton per meter squared (N/m^2). It all depends on the force and the area you're measuring.

In a liquid, the pressure comes from the weight of the fluid and the weight of the air above it, which we call the atmosphere. You feel this pressure when you swim under water. The deeper you go, the more pressure you feel. This is because there is more weight pressing down on you from above as you increase your depth.

The same principle is true for atmospheric gases. The lower in elevation you are, the more weight and pressure you experience. But even though these gases are constantly putting pressure on you, you don't feel it because your body is the same pressure as the surrounding air. Fish experience the same thing underwater - their bodies are the same pressure as the surrounding fluid, so they don't notice the constant pressure the water exerts on them.

Buoyancy is another common characteristic of fluids. This is the upward force from a fluid, which is usually felt by some object in that fluid. You float in a pool because of buoyancy, and a hot air balloon rises into the sky - also because of buoyancy.

Buoyancy in a fluid comes from the pressure of the fluid itself. An object submerged in a fluid experiences pressure from all sides. But since the pressure from below is greater (because that part of the object is deeper), it creates an unbalanced force on the object and it's pushed upward, lifting it in the fluid and against the force of gravity.

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Frequently Asked Questions

What is the study of motion in fluids?

In physics, fluids are materials that easily succumb to tangential shearing forces such as water or oxygen. The study of the dynamic movement of fluids is called hydrodynamics.

What does it mean when someone says something is fluid?

A fluid is a material that flows, and in physics describes a substance that yields to tangential shearing forces. Fluids can refer to liquids or a gases.

What are the 5 basic properties of fluid?

A fluid in physics is a material that easily succumbs to shearing forces, and the five basic properties of fluids are: surface tension, pressure, compressibility, buoyancy, and viscosity.

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