Fluids are liquids or gases, which means that
the different particles that make up fluids are
constantly bouncing about in a highly random
motion. A particle is "the atoms or molecules (for
example water molecules) that make up the liquid".
Since these atoms and molecules are constantly
moving in quick, random motions, the particles are
always bouncing off of one another.
Check out the following YouTube video for a
simulation of how the atoms in a liquid bounce
around randomly. The video depicts a simulation of
extremely low temperature liquid Argon, which is a
gas at room temperatures. Notice that the motion
of the blue ball, which is a specific atom that
has been highlighted, shows how random and "bumpy"
the liquid motion is.
The pressure on the walls of a container
results from the same random jostling of the fluid
particles that cause the blue atom to move around
in the video. At each moment in time, a
pressurized container (for example an inflated
balloon) has many more collisions occurring from
the pressurized gas on the inside than from the
lower pressure ambient atmosphere on the outside.
This difference in numbers of collisions results
in the pressure that you feel on the balloon's
surface, which resists compression when you try to
squeeze the balloon.
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