Download Pascal`s And Archimedes` Principles

What is pressure?
• Pressure is the force per unit area that
is applied on the surface of an object
• Pressure occurs at the points of contact
between the two objects
• The amount of pressure depends upon
the strength of the force and on the
size of the surface area over which the
force acts
Area and Pressure
• Area and pressure have an inverse
relationship (indirect relationship)
• This means these 2 variables change
opposite to each other (one goes up,
the other goes down, and vice versa)!
Small Area, Large Pressure
area
pressure
EXAMPLE: nail tip (small contact area, so pressure is
large enough to push apart the wood fibers)
Small Area, Large Pressure
area
pressure
EXAMPLE #2: axe splits wood (small contact area,
so pressure is large enough to push apart the wood fibers)
Small Area, Large Pressure
area
pressure
EXAMPLE #3: break board with hand (small
contact area, so pressure is large enough to break boards)
…and vice-versa
Large Area, Small Pressure
area
pressure
EXAMPLE 1: bed of nails (larger area in contact with
nail tips means pressure from any one nail is LOW)
Large Area, Small Pressure
area
pressure
EXAMPLE 2: snow shoes (larger area prevents
person from falling too deeply in the snow)
Force and Pressure
• Force and pressure have a direct
relationship
• This means these 2 variables change
together (one goes up, the other goes
up, and vice versa)!
Large Force, Large Pressure
EXAMPLE 1: blood
pressure cuff
force
pressure
Large Force, Large Pressure
EXAMPLE 2:
squeezing water
bottle with hole
force
pressure
Large Force, Large Pressure
force
pressure
EXAMPLE 3:
Hydraulic System;
Syringes
…and vice-versa
Small Force, Small Pressure
force
pressure
EXAMPLE 1: stop squeezing water bottle and
stream slows down
Small Force, Small Pressure
force
pressure
EXAMPLE 2: Accordian player stops
compressing instrument and the music stops
Small Force, Small Pressure
force
pressure
EXAMPLE 3: Nurse stops applying force to
pump and pressure reading goes down
Calculating Pressure
SI Unit for pressure is the pascal (Pa)
which is equal to 1N per 1 m2
Pre-AP:
Practice Calculations
Take out a calculator!
Example 1
P=F/A
A water glass sitting on a table weighs 4 N. The
bottom of the water glass has a surface of
0.003 m2. Calculate the pressure the water
glass exerts on the table.
P= ?
f= 4N
P = 4 / 0.003
A = 0.003 m2
P = 1,333 Pa
Example 2
P=F/A
A box that weighs 250 N is at rest on the floor. If
the pressure exerted by the box on the floor is
25,000 Pa, over what area is the box in contact
with the floor?
P = 25,000 Pa
25,000 = 250 / A
f = 250 N
A= ?
25,000 A= 250
A = 250 / 25,000
A = 0.01 m2
Fluids
A fluid is any substance that has no definite
shape and has the ability to flow.
Both liquids AND
gases are fluids
(ex. Air can flow
and has no
definite shape)
Pressure in a Fluid
What force is pulling
down on this glass?
How did the force change
when the glass was
filled with water?
How did the pressure on
the table change?
Pressure in a Fluid
The water is exerting
pressure on what two
things?
How would you calculate
the pressure from the
water glass onto the
table?
What happens to the force
and pressure when more
water is added?
Pressure and Fluid Height
Where is the pressure
from the water the
greatest?
The greater the height
of fluid above an
object, the greater the
pressure on that
object.
Pressure Increases
with Depth
When you swim underwater,
what do you notice about how
your ears feel?
depth
pressure
As height of fluid above you
increases, the weight of fluid
above you also increases.
Thus, the pressure increases
with depth.
Pressure in ALL Directions
* Pressure is on all
sides of an object
no matter what its
shape.
* Pressure is
perpendicular
(90⁰ angle) to the
surface of the
object.
Pascal’s Principle
* Pressure applied to an
enclosed fluid is transmitted
undiminished to every point
in the fluid and to the walls
of the container
* In otherwords, pressure in
constant throughout the
container
Atmospheric Pressure
* The large thickness of
the atmosphere exerts
pressure on ALL objects
at Earth’s surface.
* Your body is filled with
fluids such as blood
that also exert
pressure.
* These inward and outward pressures balance
each other
Atmospheric Pressure
altitude
air above
person
* As you go higher in
the atmosphere,
atmospheric
pressure decreases
because the amount
of air above you also
decreases.
pressure
Underwater Pressure
depth
water & air
above person
* Water pressure
increases as you
travel downward
* Water pressure is
greatest at the ocean
floor
pressure
Barometer
A barometer is a
tool that measures
atmospheric
pressure
How it works: the
height of the liquid
in the tube increases
as the atmospheric
pressure increases.
WRITE ANSWERS ONLY
FOR THE FOLLOWING
QUESTIONS
Title: “Pressure Quiz”
…and include a complete and
proper header
Question 1
What is the SI unit of pressure?
A. gram
B. liter
C. newton
D. pascal
Question 2
Pressure in a liquid _______ as
depth increases.
A. decreases
B. increases
C. remains the same
D. vanishes
Question 3
Pressure in a liquid _______ with
increased height above sea level.
A. decreases
B. increases
C. remains the same
D. vanishes
Question 4
A substance without a definite
shape, and with the ability to
flow, is known as a _______.
A. fluid
B. liquid
C. plasma
D. water
Question 5
Is air a fluid? Why or why not?
Question 6
Describe the relationship
between pressure and area and
provide a specific example.
Question 7
Describe the relationship
between pressure and force and
provide a specific example.
Question 8
What is the formula for calculating
pressure?
Pre-AP: Question 9
A student weighs 600 N. The student’s shoes are
in contact with the floor over a surface area of
0.012 m2. Calculate the pressure exerted by
the student on the floor.
Question 1
What is the SI unit of pressure?
Answer 1
The answer is D. One pascal
equals the force of 1 N applied
over an area of 1 m2.
Question 2
Pressure in a liquid _______ with depth.
Answer 2
The answer is B. You may have
experienced this in a swimming
pool. The deeper you dive, the
more pressure you feel.
Question 3
Pressure in a liquid _______ with increased height
above sea level.
Answer 3
The answer is A. The higher a
persons elevation (like on a
mountain top), the less pressure
they experience because less air
is pushing down on them.
Question 4
A substance without a definite shape, and
with the ability to flow, is known as a
_______.
Answer 4
The answer is A. Don’t confuse
this with a liquid. Gases, such as
the air you are breathing now,
are fluids but not liquids.
Question 5
Is air a fluid? Why or why not?
Answer 5
Are IS a fluid because it fits the
definition; air has no definite shape
and has the ability to flow.
Question 6
Describe the relationship between pressure
and area and provide a specific example.
Answer 6
There is an inverse relationship
between pressure and area. In other
words, as the area decreases, the
pressure increases and vice versa. The
tip of a nail is an example of this.
Question 7
Describe the relationship between pressure and force
and provide a specific example.
Answer 7
These two variables have a direct relationship. If
one goes down, so does the other and if one goes
up, the other goes up also. For example, when you
squeeze on a “holey” water bottle, the pressure
causing the water to spew out the hole is greater
when the bottle is being squeezed with a greater
force.
Question 8
What is the formula for calculating
pressure?
Answer 8
P=f/a
pressure = force / area
ADV: Question 9
A student weighs 600 N. The student’s shoes are in
contact with the floor over a surface area of 0.012 m2.
Calculate the pressure exerted by the student on the
floor.
Answer 9
P =?
f = 600 N
P = 600 / 0.012
A = 0.012 m2
P =50,000 Pa