1. 9.1 Fluids Under Pressure
Fluids always move from high pressure to low pressure
Fluids under pressure and compressed gases are used for a variety of everyday tasks
Air molecules pulled by gravity = atmospheric pressure
Air pressure increases as altitude decreases
The more air there is above, the more it
compresses the air molecules below
 Air pressure is lower at high altitudes
When humans change altitude, our
bodies try to equalize the pressure
differences by having our ears “pop”
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2. Pressure Differences
Fluids will always attempt to move from high pressure to low pressure
When we drink with a straw, we first remove
air from the closed straw, which lowers the
pressure inside. The atmosphere, having a
higher pressure, then tries to get into the
straw, and pushes the fluid up and out of the
way to try to get into the straw!
This same idea is used for many purposes, including hydraulics, water rockets and dental tools.
See page 316
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3. Liquid Pressure and Buoyancy
The pressure of fluids increases with depth
When you dive deep, you can feel more pressure
In Earth’s atmosphere and oceans, pressure also increases with depth (air behaves like as fluid as well!)
Sea level is about the “deepest” the atmosphere gets
Sea level = 1 atmosphere = kPa (kilopascals)
Top of Mount Everest = 1/3 atm = 330 kPa
From sea level, every 10 m in water depth = +1 atm
A submarine at a depth of 500 m has the equivalent of a kg object resting on every square metre!
Buoyancy refers to low density floating on high density
The amount a fluid allows
objects to float = buoyant force
See page 317
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4. Rising and Sinking
Many vehicles, including submarines, airplanes and
space shuttles all must consider pressure changes
Submarine designers must
ensure the sub is safe
design a way to change depths
When water is pumped in,
density increases = sink
When water is pumped out,
density decreases = rise
Compressed air, kept onboard, pushes out water
Convection refers to the movement of low density over top of high density fluids
See page 318
Take the Section 9.1 Quiz
(c) McGraw Hill Ryerson 2007