Name ____________________________ Date ____________________ Class ____________
Chapter 11 Forces in Fluids
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Section 1 Summary
Pressure
Key Concepts
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What does pressure depend on?
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How do fluids exert pressure?
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How does fluid pressure change with elevation and depth?
The word pressure is related to the word press and refers to a force pushing
on a surface. Force and pressure are related, but they are not the same.
Pressure is equal to the force exerted on a surface divided by the total area
over which the force is exerted. The amount of pressure you exert depends
on the area over which you exert the force.
Force
Pressure = -------------Area
The SI unit of pressure is called the pascal (Pa). One newton of force on
one square meter is one pascal of pressure (1 N/m2 = 1 Pa).
A fluid is a material that can easily flow. Fluids can change shape.
Liquids and gases are both fluids.
Fluids exert pressure against the surfaces they touch. Fluids are made up
of tiny particles called molecules. In fluids, molecules are constantly moving
in all directions. As each molecule collides with a surface, it exerts a force on
the surface. In a fluid, all of the forces exerted by the individual particles
combine to make up the pressure exerted by the fluid. Fluid pressure is the
force exerted by the fluid divided by the area over which the force is exerted.
The air in Earth’s atmosphere is also a fluid. The pressure exerted by this
gas is called air pressure or atmospheric pressure. Air exerts pressure
because it has mass. Because the force of gravity pulls down on the mass of
air, the air has weight. The weight of air produces atmospheric pressure.
In a fluid that is not moving, pressure at any point is exerted equally in
all directions. Equal and opposite pressures balance each other. You are not
crushed by the weight of the atmosphere because pressure from fluids inside
your body balances the air pressure outside your body.
Atmospheric pressure decreases as your elevation increases. At higher
elevations, there is less air pushing down from above and therefore less
weight to support. The air pressure at higher elevations is less than the air
pressure at lower elevations. Air pressure can be measured with a device
called a barometer.
Water pressure increases as depth increases. At greater depths, there is
more water pushing down from above and therefore more weight to
support. The water pressure at greater depths is more than the water
pressure at lesser depths. The total pressure underwater is a sum of the
water pressure plus the air pressure above the water.
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