FLUID STATICS - Fluid statics is the field of physics that involves the study of fluids at rest. Because these fluids are not in motion, that means they have achieved a stable equilibrium state, so fluid statics is largely about understanding these fluid equilibrium conditions. When focusing on incompressible fluids (such as liquids) as opposed to compressible fluids (such as most gases), it is sometimes referred to as hydrostatics. A fluid at rest does not undergo any sheer stress, and only experiences the influence of the normal force of the surrounding fluid (and walls, if in a container), which is the pressure. (More on this below.) This form of equilibrium condition of a fluid is said to be a hydrostatic condition. Fluids that are not in a hydrostatic condition or at rest, and are therefore in some sort of motion, fall under the other field of fluid mechanics, fluid dynamics. Consider a cross-sectional slice of a fluid. It is said to experience a sheer stress if it is experiencing a stress that is coplanar, or a stress that points in a direction within the plane. Such a sheer stress, in a liquid, will cause motion within the liquid. Normal stress, on the other hand, is a push into that cross-sectional area. If the area is against a wall, such as the side of a beaker, then the cross-sectional area of the liquid will exert a force against the wall (perpendicular to the cross section - therefore, not coplanar to it).

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Fluid Statics

By Andrew Zimmerman Jones

Fluid statics is the field of physics that involves the study of fluids at rest.

Because these fluids are not in motion, that means they have achieved a stable equilibrium state, so fluid statics is largely about understanding these fluid equilibrium conditions.

When focusing on incompressible fluids (such as liquids) as opposed to compressible fluids (such as most gases), it is sometimes referred to as hydrostatics.

A fluid at rest does not undergo any sheer stress, and only experiences the influence of the normal force of the surrounding fluid (and walls, if in a container), which is the pressure. (More on this below.) This form of equilibrium condition of a fluid is said to be a hydrostatic condition.

Fluids that are not in a hydrostatic condition or at rest, and are therefore in some sort of motion, fall under the other field of fluid mechanics, fluid dynamics.

Major Concepts of Fluid Statics

Sheer stress vs. Normal stress

Consider a cross-sectional slice of a fluid. It is said to experience a sheer stress if it is experiencing a stress that is coplanar, or a stress that points in a direction within the plane.

Such a sheer stress, in a liquid, will cause motion within the liquid. Normal stress, on the other hand, is a push into that cross-sectional area.

If the area is against a wall, such as the side of a beaker, then the cross-sectional area of the liquid will exert a force against the wall (perpendicular to the cross section - therefore, not coplanar to it).

The liquid exerts a force against the wall and the wall exerts a force back, so there is net force and therefore no change in motion.

The concept of a normal force may be familiar from early in studying physics, because it shows up a lot in working with and analyzing free-body diagrams.

When something is sitting still on the ground, it pushes down toward the ground with a force equal to its weight.

The ground, in turn, exerts a normal force back on the bottom of the object. It experiences the normal force, but the normal force doesn't result in any motion.

A sheer force would be if someone shoved on the object from the side, which would cause the object to move so long that it can overcome the resistance of friction.

A force coplanar within a liquid, though, isn't going to be subject to friction, because there isn't friction between molecules of a fluid. That's part of what makes it a fluid rather than two solids.

But, you say, wouldn't that mean that the cross section is being shoved back into the rest of the fluid? And wouldn't that mean that it moves?

This is an excellent point. That cross-sectional sliver of fluid is being pushed back into the rest of the liquid, but when it does so the rest of the fluid pushes back.

If the fluid is incompressible, then this pushing isn't going to move anything anywhere. The fluid is going to push back and everything will stay still.

(If compressible, there are other considerations, but let's keep it simple for now.)

Pressure

All of these tiny cross sections of liquid pushing against each other, and against the walls of the container, represent tiny bits of force, and all of this force results in another important physical property of the fluid: the pressure.

Instead of cross-sectional areas, consider the fluid divided up into tiny cubes.

Each side of the cube is being pushed on by the surrounding liquid (or the surface of the container, if along the edge) and all of these are normal stresses against those sides.

The incompressible fluid within the tiny cube cannot compress (that's what "incompressible" means, after all), so there is no change of pressure within these tiny cubes.

The force pressing on one of these tiny cubes will be normal forces that precisely cancel out the forces from the adjacent cube surfaces.

This cancellation of forces in various directions is of the key discoveries in relation to hydrostatic pressure, known as Pascal's Law after the brilliant French physicist and mathematician Blaise Pascal (1623-1662).

This means that the pressure at any point is the same in all horizontal directions, and therefore that the change in pressure between two points will be proportional to the difference in height.

Density

Another key concept in understanding fluid statics is the density of the fluid.

It figures into the Pascal's Law equation, and each fluid (as well as solids and gases) have densities that can be determined experimentally. Here are a handful of common densities.

Density is the mass per unit volume. Now think about various liquids, all split up into those tiny cubes I mentioned earlier.

If each tiny cube is the same size, then differences in density means that tiny cubes with different densities will have different amount of mass in them.

A higher-density tiny cube will have more "stuff" in it than a lower-density tiny cube.

The higher-density cube will be heavier than the lower-density tiny cube, and will therefore sink in comparison to the lower-density tiny cube.

So if you mix two fluids (or even non-fluids) together, the denser parts will sink that the less dense parts will rise.

This is also evident in the principle of buoyancy, that explains how displacement of liquid results in an upward force, if you remember your Archimedes.

If you pay attention to the mixing of two fluids while it's happening, such as when you mix oil and water, there'll be a lot of fluid motion, and that would covered by fluid dynamics.

But once the fluid reaches equilibrium, you'll have fluids of different densities that have settled into layers, with the highest density fluid forming the bottom layer, up until you reach the lowest density fluid on the top layer.

An example of this is shown on the graphic on this page, where fluids of different types have differentiated themselves into stratified layers based on their relative densities.

Andrew Zimmerman Jones

Math and Physics Expert

Education

M.S., Mathematics Education, Indiana University

B.A., Physics, Wabash College

Introduction

Academic researcher, educator, and writer with 23 years of experience in physical sciences

Works at Indiana Department of Education as senior assessment specialist in mathematics

Co-author of String Theory For Dummies

Member of the National Association of Science Writers

Experience

Andrew Zimmerman Jones is a former writer for ThoughtCo who contributed nearly 200 articles for more than 10 years. His topics ranged from the definition of energy to vector mathematics. Andrew is a dedicated educator; and he uses his background in the physical sciences, educational assessment, writing, and communications to advance that mission.

Andrew is co-author of String Theory For Dummies, which discusses the basic concepts of this controversial approach. String theory tries to explain certain phenomena that are not currently explainable under the standard quantum physics model.

Since 2018, Andrew has worked at the Indiana Department of Education as a senior assessment specialist in mathematics; prior to which he served as a senior assessment editor at CTB/McGraw Hill for 10 years. In addition, Andrew was a researcher at Indiana University's Cyclotron Facility. He is a member of the National Association of Science Writers.

Education

Andrew Zimmerman Jones received an M.S. in Mathematics Education from Indiana University–Purdue and a B.A. in Physics from Wabash College.

Awards and Publications

String Theory For Dummies (Wiley–For Dummies Series, 2009)

Harold Q. Fuller Prize in Physics (Wabash College, 1998)

ThoughtCo and Dotdash

ThoughtCo is a premier reference site focusing on expert-created education content. We are one of the top-10 information sites in the world as rated by comScore, a leading Internet measurement company. Every month, more than 13 million readers seek answers to their questions on ThoughtCo.

For more than 20 years, Dotdash brands have been helping people find answers, solve problems, and get inspired. We are one of the top-20 largest content publishers on the Internet according to comScore, and reach more than 30% of the U.S. population monthly. Our brands collectively have won more than 20 industry awards in the last year alone, and recently Dotdash was named Publisher of the Year by Digiday, a leading industry publication.

https://www.thoughtco.com/fluid-statics-4039368

YIN AND YANG - In Chinese philosophy, yin and yang describes how seemingly opposite or contrary forces may actually be complementary, interconnected, and interdependent in the natural world. Yin and Yang is two halves that together complete wholeness. Yin and yang are also the starting point for change. When something is whole, by definition, it’s unchanging and complete. So, when you split something into two halves – yin/yang, it upsets the equilibrium of wholeness. Both halves are chasing after each other as they seek a new balance with each other.

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Yin And Yang

What is yin and yang?

factsinbrief.com

Yin Yang is perhaps the most known and documented concept used within Taoism.

In Chinese philosophy, yin and yang describes how seemingly opposite or contrary forces may actually be complementary, interconnected, and interdependent in the natural world.

Yin and Yang is two halves that together complete wholeness.

Yin and yang are also the starting point for change. When something is whole, by definition, it’s unchanging and complete.

So, when you split something into two halves – yin/yang, it upsets the equilibrium of wholeness. Both halves are chasing after each other as they seek a new balance with each other.

The word Yin comes out to mean “shady side” and Yang “sunny side”. 

Yin Yang is the concept of duality forming a whole.

We encounter examples of Yin and Yang every day.

As examples: night (Yin) and day (Yang), female (Yin) and male (Yang).

Over thousands of years, quite a bit has been sorted and grouped under various Yin Yang classification systems.

The two opposites of Yin and Yang attract and complement each other and, as their symbol illustrates, each side has at its core an element of the other (represented by the small dots). Neither pole is superior to the other.

   

https://www.factsinbrief.com/en/2018/06/14/what-is-yin-and-yang/

ATMOSPHERIC PRESSURE - Air consists of an incredibly large number of molecules. Although these molecules are very small, they are ‘respectable’ enough to have some weight of their own. We are talking about a large number of air molecules that are pretty light individually, but become quite ‘weighty’ in great numbers. Atmospheric pressure acts in basically the same way. Thousands of air molecules weigh down on you all the time. In fact, the standard value of atmospheric pressure is 14.7 pounds per square inch at sea level. To put that in perspective, it’s like holding a small car on your head all the time. The air particles around you exert a certain amount of pressure on your entire body, but what’s interesting is that the same amount of pressure is exerted back onto the air molecules by the insides of the body, thereby achieving a state of equilibrium.

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Atmospheric Pressure

Why Don’t We Get Crushed By Atmospheric Pressure?

Ashish  

Let me tell you right from the start, we’re under a lot of pressure.

As humans, there are many types of pressure that one has to look out for in their lives, but there is this one pressure that bears down on every single human being on Earth, including Hulk, Iron Man and Captain America.

The air that surrounds us may seem to be absolutely weightless, but it’s far from being so. Why do we assume that air is weightless?

Does it not have the right to some weight of its own? After all, the atmosphere is a part of the planet, and it contains a number of gases that are present in differing amounts.

The Atmosphere

Air consists of an incredibly large number of molecules. Although these molecules are very small, they are ‘respectable’ enough to have some weight of their own.

Although it’s true that a single molecule is amazingly light and seemingly non-existent, you may feel a bit amazed when I tell you that many air molecules are weighing down on you right now, at this very moment.

Essentially, we are talking about a large number of air molecules that are pretty light individually, but become quite ‘weighty’ in great numbers.

How does it feel when you lift something very heavy on your shoulders or head? You feel a pressure weighing down on you, right?

Atmospheric pressure acts in basically the same way. Thousands of air molecules weigh down on you all the time.

In fact, the standard value of atmospheric pressure is 14.7 pounds per square inch at sea level. To put that in perspective, it’s like holding a small car on your head all the time.

So, how come we don’t feel it?

Equilibrium: The Great Equalizer

The air particles around you exert a certain amount of pressure on your entire body, but what’s interesting is that the same amount of pressure is exerted back onto the air molecules by the insides of the body, thereby achieving a state of equilibrium.

If the human body were an empty shell, i.e., if it didn’t contain the fabulous assortment of organs, bones, muscles, blood and other such things, then it would have popped like a tin can under our atmospheric pressure. However, that doesn’t happen.

This essentially implies that there is a certain equalization of pressures involved in this case, which is why there is no pressure difference and why we don’t feel ‘burdened down by air’.

The pressure of air that is present outside your body is the same as that of the air present ‘inside’ your body.

The air that is constantly present in your lungs, ears and nose has the same atmospheric pressure as the air on the outside of your ears, nose, and chest.

Since there is no pressure difference, we don’t feel anything at all, as far as atmospheric pressure is concerned.

When There is a Pressure Difference…

You understand by now that you don’t feel the atmospheric pressure due to an absence of a pressure difference between the external air and the air that’s inside your body, which we’ll call ‘internal air’.

However, what if there is a pressure difference? Will something truly awful happen? How rare is the occurrence of a pressure difference?

You may be a wee bit surprised to know that the occurrence of these pressure changes is actually quite commonplace. You may experience it often if you are a frequent flier. Getting my drift?

You know that funny feeling when your eardrums seem to ‘close’ themselves when your plane takes off or lands? What about when you enter a long tunnel or exit it?

The popping of the ear is directly associated with a change in the external and internal air pressure.

You see, when your aircraft is on the runway, the pressure in your ear is the same as the pressure in the aircraft’s cabin.

However, as you take off and go high into the skies, an inequality in the external and internal pressure develops, and your ears seem to shut themselves off.

Having a conversation in such conditions isn’t advised, especially if you’re trying to make personal or professional ties.

There are many other instances of changing air pressure, most of which seem to primarily affect the nose and ear.

There is no ‘cure’ for it, per se, but there are certain methods that could definitely help you open your eardrums and get the pressure back to normal.

One such technique involves closing both the nostrils and mouth, and then gently blowing out air through the nose. Big yawns also help to ‘un-pop’ the ears.

We can’t be thankful enough for the wonders that nature presents all around us.

It works in mysterious ways and balances every variable in its ambit impressively to ensures that all the natural conditions are stacked up in a way that sustains and progresses life on Earth.

Ashish is a Science graduate (Bachelor of Science) from Punjabi University (India). He spends a lot of time watching movies, and an awful lot more time discussing them. He likes Harry Potter and the Avengers, and obsesses over how thoroughly Science dictates every aspect of life… in this universe, at least.

https://www.scienceabc.com/nature/dont-get-crushed-atmospheric-pressure.html