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Fluid Mechanics

Fluid mechanics also sometimes referred to as fluid dynamics is a discipline within the field of applied mechanics.  Mainly fluid mechanics is the study of both liquids and gases that are either at rest or in motion.  Some phenomena of study will naturally occur in nature while others could be man made. As a matter of fact there are many aspects of our lives where fluids affect us directly or indirectly.

What is a Fluid?

The first thing that you must know is what is a fluid?  Or in other words how does a fluid differ from a solid.  Well first lets look at the three basic states of matter.  Not including plasma, the three basic states of matter are gas, liquid, and solid.  Out of these three forms of matter a gas and a liquid are fluid.  So why isn’t a solid a fluid.  First, lets describe the differences that we can physically see and touch.  We know that a solid is in fact solid.  This means that we cannot move through it.  A fluid, however, will allow us to move through it, liquid or gas.  Next, a solid holds its shape.  Fluids on the other hand like to take the shape of their container.

So why does the above observations take place between a Solid and a Fluid?  Generally speaking, it all has to do with the molecular structure of the molecules of a fluid vs a solid.  A solid is a solid because of how close the molecules are to each other.  This in turn allows large cohesive forces between the molecular bonds of a solid allowing it to keep its shape.  However, when matter becomes fluid, the distance between the molecules increases.  As a result the cohesive forces become weaker, allowing the molecules to move within the fluid.  In turn, since the molecules of a fluid can move, the fluid will deform without much resistance.

Molecular structure Solid, liquid, gas

Specifically a fluid by definition is a “substance that deforms continuously when acted on by a shearing stress of any magnitude”.  A shear stress is created when there is a tangential force acting on the surface of the fluid.

Gas or Liquid

As mentioned above a fluid can be either a gas or a liquid.  Both gas and liquid will take the shape of there container, and both will deform at the slightest shear force.  However, they are still different.  A liquid unlike a gas is considered incompressible.  This means if you were to put pressure on a liquid its volume will not change.  A gas, however, will readily change its volume in accordance with the pressure placed on it.

Next, as mentioned above, both liquids and gas will take the shape of their containers.  However,  a gas will increase or decrease its volume to fill the whole container.  A liquid on the other hand will fill the container up to its total volume.

These difference between a liquid a gas, as with a solid, all comes down to the molecular structure of the molecules.  A liquids molecules will not rigidly connect to each other like a solid will, but instead will flow around each other, but stay near each other.  A gas molecule, however, has so much molecular energy within them that they go where they please regardless of what the other particles are doing.  This is why when you compress a gas the pressure increased, since there are more gas molecules making contact with the sides of the container.

Not quite Fluid or Solid

Finally, there are some materials that can act like a fluid and a solid.  Some examples of these substances would be tooth paste, putty, and tar.  To name a few.  If you were to place a shear force on these substance they would continuously deform.  This causes the substance to act a fluid.  On the other hand these substance will hold there shape when a force is not applied to them, which means they can withstand a slight shear force.  This is where these substances act like a solid since a fluid will take the shape of its container.  These materials do not fall under the classic study of fluid mechanics, instead the study of these materials is known as rheology.

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