Ocean waves off the coast of Mexico
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copyright 2002 University Corporation for Atmospheric Research
What is a Fluid?
A fluid is anything that would spill or float away if it
weren't in a container (unless it's big enough to be held
together by gravity like a star). If you can stir it up with
a spoon or blow it through a straw, it's a fluid. Water is
a fluid and so is air. In fact, all liquids and gases are fluids.
In space and inside stars there's also another kind of fluid
called a
plasma.
The molecules
in a solid are stuck together, but in a fluid they're
free to move past each other. So if you had very small hands
you could push one molecule of a fluid one way and another molecule the other
way and off they would go in the direction you pushed them.
Much of the universe is made of fluid, including Earth's
atmosphere
and oceans,
giant planets like Jupiter,
stars like the Sun, and
huge clouds of
gas and dust in space. Even rock and metal can be fluid
if they're hot enough; that's what happens
deep inside the Earth.
Fluid dynamics (also called fluid mechanics) is the science of how fluid
moves around. A fluid in motion is called a flow.
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