Download 11.1 Pressure

Section 1: Pressure
1. What is Pressure?
a. It refers to a force exerted over an area on the surface of an object.
i. Due to gravity, your feet exert a force on the surface of Earth—your feet
exert pressure on the ground
b. Pressure and Area
i. Pressure decreases as the area over which a force is distributed increases
1. The larger the area over which the force is distributed, the less
pressure is exerted
c. Calculating Pressure
i. Pressure = Force / Area
1. Pressure equals force divided by area
2. Force is measured in newtons (N)
3. Area is measured in square meters (m2)
4. The SI unit of pressure is the newton per square meter (N/m2)
a. This unit of pressure is also called the pascal (Pa)
i. 1 N/m2 = 1 Pa
2. Fluid Pressure
a. What causes fluid pressure?
i. Particles in a fluid constantly move in all directions
1. The particles collide with each other and with any surface that they
meet
ii. All of the forces exerted by the individual particles in a fluid combine to
make up the pressure exerted by the fluid
iii. Fluid pressure is the total force exerted by the fluid divided by the area
over which the force is exerted
b. Air Pressure
i. 100 kilometers of fluid, air, surrounds Earth
ii. Air is a mixture of gases
1. These gases press down on everything on Earth’s surface, all the
time
iii.Air exerts pressure because it has mass
1. Because gravity pulls down on this mass, air has weight
2. The weight of the air produces air pressure, or atmospheric
pressure
c. Balanced Pressure
i. Hold out your hand, palm up. You are holding up air with atmospheric
pressure about equal to 10.13 N/cm2
1. The weight supported by the surface area of your hand is about
1,000 newtons, or about the same weight as that of a large
washing machine!
ii. In a stationary fluid, the air is pressing in all directions so they balance
each other.
iii. Balanced Pressure explains why the air pressure around us does not crush
us
1. We have fluids in our body that exert outward pressure
iv. When air pressure becomes unbalanced
1. A can full of air has balanced air pressure inside and outside the
can
2. When the air is removed, the can crushes
3. Variations in Fluid Pressure
a. Atmospheric Pressure and Elevation
i. As your elevation increases, atmospheric pressure decreases
ii. The reason our ears “pop”
1. Has to do with changing air pressure
2. At higher elevations, there is less air above you and therefore less air
pressure
3. When the air pressure outside your body changes, the air pressure
inside adjusts, but more slowly
a. The air pressure behind your eardrums is greater than it is in the
air outside.
b. Your body releases this pressure with a “pop” as pressures balance
b. Water Pressure and Depth
i. Fluid pressure depends on depth
ii. The deeper you go into a body of water, the greater the pressure
iii.Water pressure increases with depth
iv.Just like air, you can think of water pressure as being due to the weight of
the water above a particular point
v. The total pressure at a given point beneath the water results from the
weight of the water plus the weight of the air above it
c. Measuring Pressure
i. Barometer: instrument used to measure atmospheric pressure
1. Two types: A mercury barometer and an aneroid barometer
a. Aneroid barometer
i. The kind you usually see hanging on a wall
ii. Used by weather forecasters
iii.Rapidly decreasing atmospheric pressure usually means a
storm is on its way
iv.Increasing atmospheric pressure is a sign of fair weather
v. Readings may be read in millimeters, inches, or millibars
vi.Ex: The barometric pressure at sea level may be reported as
760 millimeters, 29.92 inches, or 1,013.2 millibars

What is the SI unit for pressure?

What two factors does pressure depend on?

How do fluids exert pressure?

Why must an astronaut wear a pressurized
suit in space?