13.1 & 13.2
Pressure
Pressure
 Pressure is the result of a force distributed over an
area.
• The unit of force is the newton (N).
• The unit of area is the square meter (m^2).
• The SI unit of pressure is the pascal (Pa). A pascal is one
newton per square meter (N/m^2)
Pressure Problem
 If the area of a box touching the ground is 1.5 square
meters, and its weight is 2700 newtons, what pressure
does the box exert on the ground?
Pressure in Fluids
 Water pressure
increases as depth
increases. The pressure
in a fluid at any given
depth is constant, and it
is exerted equally in all
directions.
Pressure in Fluids
A fluid is a substance that assumes the shape of its
container.
• Both liquids and gases are fluids.
• Water, oil, gasoline, air, and helium are examples of
fluids.
Fluids vs Gases
A. Particles in a liquid are tightly packed together
but are able to slide past one another.
Fluids vs Gases
B. Particles in a gas are far apart and travel in
straight lines until they collide with another
particle or object.
Fluid Pressure
Water in a glass is in contact with the walls and bottom
of the glass,
Exerts pressure on these surfaces.
For a fluid that is not moving:
depth and the type of fluid determine the pressure the
fluid exerts.
Fluid Pressure
 Each vase holds a different amount of liquid. The
liquid levels are all the same because pressure depends
on depth, not amount.
Atmospheric Pressure
 The layer of gases surrounding a planet is known as its
atmosphere.
 All of the planets in our solar system have some form of
atmosphere.
 The weight of an atmosphere creates atmospheric pressure
at the planet’s surface.
Atmospheric Pressure
 Which planet has the highest pressure?
Atmospheric Pressure
 The bar is another unit of pressure (1 bar = 101.3 kPa).
Convert each of the given pressures into bars.
Atmospheric Pressure
 How much force is exerted on a 2.00-square-meter
area of Venus’s surface?
Atmospheric Pressure
 On which planet would a balloon filled with a given
quantity of helium have the smallest volume?
Air Pressure
 Air pressure decreases as the altitude increases.
 Earth’s atmosphere exerts pressure of about 101 kPa at
sea level. Why aren't you crushed by air pressure?
Atmospheric Pressure
 The pressure inside your body
balances the air pressure
outside. The balanced forces
cancel, resulting in a net force
of zero.
Pascal’s Principle
 According to Pascal’s principle:
 A change in pressure at any point in a fluid is
transmitted equally and unchanged in all directions
throughout the fluid.
Transmitting Pressure
Pascal’s Principle
A. The forces exerted against
the walls of the container
are equal at a given depth.
B. When squeezed, the
pressure is transmitted
equally throughout the
fluid.
Pressure vs Depth
At any given depth, equal pressures act against all
points on the inside of the bottle.
When the bottle is squeezed, the pressure still
increases with depth.
The pressure increases equally throughout the water.
Hydraulic Systems
 A hydraulic system is a device that uses pressurized
fluid acting on pistons of different sizes to change a
force.
 In a hydraulic lift system,
 an increased output force is produced because a
constant fluid pressure is exerted on the larger area of
the output piston.
Hydraulics
 The truck uses hydraulic-powered struts to lift its load.
The larger area of the output piston produces the
increased force.
Hydraulics
When an input force is applied to the small piston in
a hydraulic system, the piston pushes against the
fluid sealed in the system.
• The pressure produced by the small piston is
transmitted through the fluid to the large piston.
• The pressure on both pistons is the same.
• Because the output pressure acts on a much larger area,
the output force is larger.
13.2
Bernoulli’s Principle
 According to Bernoulli’s principle, as the speed of a
fluid increases, the pressure within the fluid
decreases.
Experiment
Try this simple experiment.
• Hold a sheet of paper by its top corners.
• Position the paper directly in front of your mouth.
• Blow over the top surface of the paper.
• Can Experiment
Bernoulli’s Principle
As the air blows across the top of the paper, the
pressure exerted by the air decreases.
• The motionless air below the paper exerts a greater
pressure.
• The difference in pressure forces the paper upward.
Bernoulli’s Principle
Wings and Lift
Bernoulli’s principle
explains the ability of birds
and airplanes to fly.
• Air traveling over the top of
an airplane wing moves
faster than the air passing
underneath.
• The pressure difference
between the top and the
bottom of the wing creates
an upward force known as
lift.
Bernoulli’s Principle
 Air flowing over the top of the wing is diverted up and
over the wing’s curved surface.
Similar Concept
 The downward force created by the spoiler on a race
car pushes the tires down onto the road, giving the car
better traction.
Other
Uses
Spray Bottles
Pressure differences between the solution chamber and
the moving stream of water draw the solution into the
stream.