Fluid Pressure
Chapter 13.1
Pressure
What is the definition of pressure?
The result of a force distributed over an area
The book talks about the difference in
pressure between a bicycle seat and a
theater seat. Give one original idea
regarding this same idea:
Snowshoes vs. high heels
What is the formula for finding
pressure?
Pressure = force/area
Pa = N/m2
What is the SI unit for pressure? Why
do we usually use the prefix “kilo” in
front of this unit?
Pascal (kilopascal)
A pascal is too small for normal use
Pressure in Fluids
What is the scientific definition of
fluid?
A substance that assumes the shape of its
container
What phases of matter are considered
fluids?
Liquids & gasses
What things affect water pressure?
Depth
As you get deeper in the water, pressure
increases
Type of fluid
What things do not affect water
pressure?
Shape of container
Area of the bottom of a container or its
surface
Amount of fluid (volume)
Air Pressure and the
Atmosphere
What is air pressure at sea level on
Earth?
101 kPa
Relate air pressure and altitude:
As you get “deeper” in the air, the pressure
increases
Why are you not affected by the 1000+
Newtons of force pushing down on you
right now?
The inside of your body also exerts
pressure
(an equal and opposite force)
The forces are balanced = net force of
zero
Review
1. What must you know to calculate
pressure?
Force and Area
Review
2. What is the relationship between the
depth of water and the pressure it exerts?
Water pressure increases as depth
increases.
Review
3. How is pressure distributed at a given
level in a fluid?
At a given depth, pressure is constant and
exerted equally in all directions.
Review
4. How does the pressure exerted by the
atmosphere change as altitude increases?
Atmospheric pressure decreases as altitude
above sea level increases.
Review
5. Why don’t you feel the pressure exerted
by the atmosphere?
You cannot feel atmospheric pressure
acting on your body because the pressure
inside your body balances the pressure
outside.
Review
6. Some deep-sea fish have been known to
explode as they are brought to the ocean’s
surface. How do pressure changes cause
this to happen?
Deep-sea fish have high internal body
pressures in order to counteract the extreme
water pressure that exists where they live.
When the fish is suddenly brought to the
surface, where the pressure is much less, its
internal pressure can cause it to rupture or
explode.
Review
7. The 500 N student stands on one foot. A
750 N student stands on two feet. If both
students wear the same size shoe, which
exerts the greater pressure?
Do the math!
The 500 N student.
Review
8. A circus performer on a pair of stilts
exerts a pressure of 32 kPa on the
ground. If the performer stands on one
stilt, what pressure does the stilt exert on
the ground?
Twice as much, because the area is twice as
small.
Review
9. A book with a weight of 12 N rests on
its back cover. If the back cover
measures 21 cm by 28 cm, how much
pressure does the book exert?
200 Pa
Forces and Pressure in Fluids
Chapter 13.2
Transmitting Pressure in a Fluid
Pascal’s Principle
State Pascal’s Principle:
The change in pressure anywhere in a fluid
is transmitted equally & unchanged through
all the liquid (in all directions)
Hydraulic Systems
Describe a hydraulic system:
A device that uses pressurized fluid acting on
pistons of different sizes to change a force.
If Pascal’s Principle is true, how can a
hydraulic system produce an increased
output force?
Because a constant fluid pressure is exerted
on the larger area of the output piston.
Force = Pressure x Area
Bernoulli’s Principle
State Bernoulli’s Principle:
As the speed of a fluid increases, the
pressure within the fluid decreases.
How is a fluid’s speed related to the
pressure it exerts?
Slow or no speed = great pressure
High speed = lower pressure
Wings & Lift
Explain lift:
An upward force is created by having a low
pressure over a high pressure.
How does a car’s wing create a
downward force?
It is an upside-down wing.
Note: Wing & Spoiler are
different on a car
Spray Bottles
How do spray bottles work?
Check out figure 9 (pg. 397)
Review
1. Describe Pascal’s principle in your own
words:
When there’s a change in pressure
somewhere in a fluid, it will spread evenly
throughout the fluid (trying to even out).
Review
2. How is a hydraulic system able to
increase force?
The increased output
force is produced
because the fluid
pressure is exerted on
the larger area of the
output piston.
Review
3. How are fluid speed and fluid pressure
related?
As the speed of a fluid increases, the
pressure within the fluid decreases.
Review
4. How does an airplane wing produce lift?
Bernoulli’s principle:
Air travels faster over the top of the wing,
producing low pressure. Slower air on the
bottom of the wing creates higher pressure.
Review
5. Water is nearly incompressible. Air is
easily compressed. Predict what will
happen when an air-filled balloon is
pulled to the bottom of a swimming
pool.
The air-filled balloon will be compressed.
Review
6. When cars pass one another in opposite
directions on the highway, they tend to
be forced together. Use Bernoulli’s
principle to explain why this happens.
Review
7. Two table tennis balls are suspended
from strings so they hang at the same
height with a small space between them.
When a hair dryer is used to blow air
between them, they come together and
touch. Explain why this occurs:
Buoyancy
Chapter 13.3
Buoyant Force
What is the definition of buoyancy?
The ability of a fluid to exert an upward
force on an object.
It is what seems to cause a “loss of weight”
of an object in a fluid.
Does every object in a fluid experience
buoyancy?
Yes, even if it sinks, there’s still some
upward force.
What is a buoyant force?
An upward force, which acts in the opposite
direction as gravity.
It makes it easier to lift things in fluid.
What causes a buoyant force?
Forces from fluid pressure act on objects in
all directions, but most from the bottom,
pushing the object up.
See Figure 10, page 400.
The net force is up (using vector addition, of
course!)
Archimedes’ Principle
The buoyant force on an object is equal
to the weight of the fluid displaced by the
object.
When an object is submerged, it
pushes aside (displaces) a volume
of fluid equal to its own volume.
(the part underwater)
Density & Buoyancy
How are density and buoyancy related?
Density = mass/volume (g/cm3)
Buoyancy = weight of the volume of
displaced fluid (N)
Relate an object’s density to whether or
not it will float:
Objects less dense than the fluid will float
Objects more dense than the fluid will sink
Relate an object’s weight and buoyant
force to whether or not it will float:
Objects that weigh less than the buoyant
force will float
Objects that weigh more than the buoyant
force will sink
Suspended
When an object has the same density as
the fluid it’s in, it will be suspended.
Buoyant force = weight
Not sinking, not floating
Sinking
If the object’s weight is greater then the
buoyant force, it will sink.
If a ship takes on water, it gets heavier
and will sink.
Floating
The shape of a ship allows it to displace
enough water to float.
(Increasing displacement increases buoyant
force)
It’s easier to float in dense liquids,
because there’s a greater buoyant force.
(salt water is more dense than fresh water)
Review
1. How does buoyancy affect the apparent
weight of an object in a fluid?
Review
2. What determines if an object will float
or sink in a fluid?
Review
3. How does Archimedes’ principle relate
the buoyant force acting on an object
and the volume of fluid displaced by the
object?
Review
4. How is the density of a floating object
related to the density of the fluid it is
floating in?
Review
5. An empty oil tanker displaces enough
water to support its weight. Why doesn’t
the tanker sink when laded with
thousands of tons of oil?
Review
6. A small object is able to float at any
level when placed in water. What does
this observation tell you about the
object’s density?
Review
7. A 350 N block of wood is thrown into a
lake, where it floats. What is the buoyant
force acting on it?