FORCES IN FLUIDS

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FORCES IN FLUIDS

• CHAPTER 11

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Section 11-1Pressure
Pressure - related to the word press - refers
to the force pushing on a surface
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Force Pressure
The size of an area over which pressure is
exerted makes a difference.
No snowshoes
Snowshoes
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Calculating Pressure
This formula summarizes the relationship between
pressure, force, area. Force is measured in N -
newtons Area is measured in square units (meters)
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Pascals
So pressure is measured in the unit called the
pascal (Pa) 1 N/m2 1 Pa
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Force in a Fluid
The diagram shows that the fluid force on 1
square cm is 12 N. What is the
pressure? 12N/1cm 12 N/cm2
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What would happen if you made the area affected
by the force smaller or larger?
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You can produce a lower pressure by increasing
the area a force acts on. You can produce a
higher pressure by decreasing the area a force
acts on.
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Fluid Pressure
A fluid is - - a liquid or a gas - a substance
that will flow easily - able to change
shape -exert pressure on surfaces they touch
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Molecules of Fluids
Fluids are made up of molecules - (extremely
tiny particles. 1 L of water contains about 33
trillion trillion molecules. 33 24 0s
!!!) - in constant motion in all directions -
constantly collide with each other and
surfaces they contact
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Molecules of Fluids
All of the force exerted by the individual
molecules in a fluid add together to make up the
pressure exerted by the fluid.
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Fluid Pressure Around Us
You are surrounded by a fluid that presses down
on you all the time. The atmosphere. (You have
the weight equivalent to a washing machine on
you!) This is called air pressure or atmospheric
pressure.
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This diagram explains why we dont feel air
pressure.
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Air exerts pressure because it has mass. Each
cubic meter of air 1 kg - the force of gravity
on this air produces pressure form the weight of
the air. The atmosphere is over 100 km high.
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Air pressure at sea level is about 10.13 N/cm2
Your hand is about 100 cm2 so the total force
on your hand is about 1000 N.
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Balanced Pressures
How can you support the weight of the
atmosphere? Remember Pressure in a fluid is
exerted equally in all directions - so if there
is 10.13 N/cm2 on the top of your hand there is
also 10.13 N/cm2 on the bottom.
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Balanced Pressures
So why arent you crushed by the
atmosphere? Balanced pressures - pressure
inside of your body is equal to that
outside. Some of your body has air spaces (lungs,
sinuses, ears) and part is filled with liquids.
(cells, blood)
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Look at what happens to this can if we pump all
of the air out of it.
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Why did it crush like this?
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Balanced Pressures
When the air is removed from the inside of the
can, there is no air pressure pushing on the
inside. So the air pressure on the outside has
nothing to balance its push (nothing on the
inside is pushing out) and the can crushes inward.
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Variations in Fluid Pressure
Pressure elevation - - air pressure decreases
as elevation increases - air pressure at a
given point results from the weight of air
from above. At higher elevations, there is
less air above so less weight. This is why
your ears pop in an elevator or airplane. If the
pressure in your ears is higher than outside,
your body equalizes the pressure and this is the
pop you hear.
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Pressure Depth
Fluid pressure depends on depth. Water pressure
increases with depth due to the weight of the
water from above.
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Pressure of water - also remember the weight of
the air is pushing on top of the water. The
deepest parts of the ocean may have 1000 times
the pressure of the air.
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You can feel the higher water pressure at the
bottom of a pool.
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Section 11-2Transmitting Pressure in a Fluid
Pascals Principle - when force is applied to
confined fluid, an increase of pressure is
transmitted equally to all parts of the fluid.
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Pascals Principle - note the magnitude of the
force when the stopper is pushed
further in.
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Force Pumps
A force pump causes a fluid to move from one
place to another by increasing the pressure in
the fluid. Your heart consists of two force
pumps. One pumps blood to the lungs the other
returns the blood to the heart.
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Using Pascals Principle
A force applied to one piston increases the
pressure in the fluid.
Note the pistons are equal in area. So they
experience the same force.
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The left piston has the area of 1 square meter
the right piston has an area of 20 square
meters. Then if you pushed down on the left
piston with the force of 500 N, you increase in
pressure on the fluid is 500 N/m2. Remember
pressure force/area p 500N/1m2
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This means there is 500 N of force on every
square meter. Since the surface area of the
right piston is 20 square meters, what is the
total increase of force?
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20,000 N The left pistons force has been
increased or multiplied by 20 times!
Depending on the size of the pistons, you can
multiple force by any amount.
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Hydraulic Systems
Hydraulic systems are designed to take advantage
of Pascals Principle. A hydraulic system
multiplies a force by applying the force to a
small surface area. The increase of pressure is
then transmitted to another part of a confined
fluid, which pushes on a larger surface area.
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The brakes in your car is a common hydraulic
system. This allows a person to push on a small
pedal and stop a large vehicle.
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Sea stars use hydraulics in their tube feet to
move around and to cling to rocks.
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Section 11-3Floating and Sinking
Why does something that floats also
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sink?
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Buoyancy
Buoyant force - a force that water exerts. The
buoyant force acts in the upward direction,
against the force if gravity, so it makes an
object feel lighter. Objects feel less heavy in
water.
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Buoyant Force
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Pressure is exerted on all surfaces of a
submerged object. Since the pressure in a fluid
increases with depth, the upward pressure is
greater at bottom of the object than the pressure
at the top. The result is a net force in the
upward direction. This is BUOYANT FORCE.
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Archimedes Principle
States that the buoyant force on an object is
equal to the weight of the fluid displaced by the
object. Archimedes was an ancient Greek
mathematician.
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This is a demonstration of displacement. Explain
what you see.
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A. B.
C.
Compare the direction of the buoyant force to the
direction of gravity. Compare the size of the
arrows. (Magnitude of force.)
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A. B.
C.
A. Forces are balanced. B. Weight is greater
than the buoyant force. C. The amount of water
displaced is to the weight of the object
so now the forces are balanced.
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Density
Density Mass / Volume
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In terms of density, explain what A, B, and C did.
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Will a rubber washer sink or float in corn oil?
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This person is floating in the Great Salt Lake.
Why is he floating so high in the water?
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Buoyancy and density.
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The solid steel cube sinks (note its small
buoyant force.) The same amount of steel spread
into different shapes has different buoyant force
water displacement.
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How does a submarine work??
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How do these balloons float sink?
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Applying Bernoullis Principle
Bernoullis Principle - states that that the
pressure exerted by a moving stream of fluid is
less than the pressure of the surrounding fluid.
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Try this
Take a piece of paper. Place 3 cm of a long
edge between the pages of a text book. The
paper should mostly hang free. Hold the book
vertically so the paper hangs away from you. Blow
air over the top of the paper. WHAT HAPPENS??
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This is Bernoullis Principle in action. Blowing
over the paper makes a moving stream the
pressure on top will be lower. The pressure under
the paper will be higher so the paper will rise.
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Chimneys work using Bernoullis Principle. How?
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Air blowing over the top of the chimney causes
the air pressure to be lower at the top of the
chimney. This will make the smoke rise up and
out of the chimney instead of filling the room.
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Wings Bernoullis
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As air travels over the wing, it is forced to
move farther therefore moves faster. So the
air moving over the top has less pressure. The
higher pressure air underneath pushes UP .
This is called LIFT.
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Spoilers
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Why does the shower move into the
shower? Bernoullis can explain this. What is
the moving fluid in the shower?
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What is suction? Suction results from unequal
air pressure.
Examples using a straw, a vacuum cleaner,
medicine droppers, suction cups, your lungs.
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The atomizer.
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Bernoullis at work. A squeeze of the bulb
causes air to move quickly over the top of the
tube which extends down into the liquid.The
greater pressure in the flask causes the liquid
to spray out.
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THE END