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Pressure
For every force there is an equal & opposite force - when you push down on the water, the water pushes back on you
The “push” or force per unit area that is exerted on an object is called pressure
Pressure = force/area Units?
Using the same force which has more
pressure, finger or palm?
How would you rescue an ice skater that fell
through the ice?
Barometer
• Used to measure atmospheric pressure
h
Air Pressure
Standard air pressure = 10.13 N/cm2 or ~ 15 lbs/in2
Area of your back = 1000 cm2
Force = (10.13)(1000)=10,130 N against your back
Why don’t you tip over?
Air pressure demo
What happens to air pressure at higher altitudes?
Why?
What would happen if a bag of chips sealed at sea level were
taken to the top of a mountain?
What would happen if a bag of chips sealed at sea level were
taken to the top of a mountain?
Suction “This Sucks! I'm under so
much pressure! (Impossible... Science CAN'T suck!)”
Before using the straw the air pressure and water pressure are equal
When you “suck” on a straw you remove most of the air inside the straw
Air pressure outside is greater than the air pressure inside the straw
Outside pushes down on the surface of the drink which pushes the drink up through the straw
Straw
Explain the following pressure demonstrations
• Suspended card• Egg in bottle• Erlenmeyer flask with balloon• Boiling water in vacuum• Straw in water• Crushed can• Blowing up balloons in bottles• Crushed egg• “Pirates of the Caribbean”
What happens to pressure in a liquid as the depth is
increased?
Buoyancy Force of a fluid that pushes an object up Acts against the weight of an object If an object floats. . .
the buoyant force is equal to
the weight of the object If an object sinks . . .
the buoyant force is less than
the weight of the object
Force of gravity or weight of object
Buoyancy
Force of gravity or weight of object
Buoyancy
Archimedes’ principle The buoyant force on an object is equal to
the weight of the fluid displaced by the object
Use Archimedes’ Principle to compare the weight of water displaced and the weight of the object if an object floats. If an object floats the weight of the object
_______ the weight of water displaced
Archimedes’ PrincipleSilly putty in a ball sunk
Weight of silly putty
Weight of displaced water>
Silly putty flattened floated
Weight of silly putty
Weight of displaced water=
How much water is displaced this time: more, less or the same as the first time?
Why do people seem to weigh less under water?
Which displaces more water? Which has a better chance of floating?
Why?
50g50g
Major cargo ships sail up the Saint Lawrence seaway from the Atlantic ocean to the great
lakes. If their ballast tanks are not adjusted, will the ships ride higher or lower when they
move from the ocean to the freshwater seaway?
Bernoulli’s principle
• The pressure in a moving stream of fluid is less than the pressure in the surrounding fluid
Explain each of these Bernoulli’s observations
• Straw in water – blow over top of straw• Paper between two books• Ball with air source• Toy straw• Finding Nemo• Toy helicopter
Airplane wing
Top surface is longer than bottom surface air traveling on top goes farther distance
than air on bottom, but air particles reach back at the same time
air above the wing is moving faster than the air under the wing
Forces on airplane
• Lift - upward force on the wings• Weight - downward pull of gravity• Thrust - forward force needed to move
through the air• Drag - fluid friction
When an airplane is in level flight, lift is equal to weight of
the plane. What happens when the lift is greater than
the weight of the plane?
Suspended Card
• Air pressure is more than the weight of the water enabling the card to hold the water up in the cup
Egg in Bottle• Flame uses oxygen
that was in the bottle and puts it in the ashes creating less molecules in the air and a low pressure in the bottle
• The higher pressure outside the bottle pushes the egg into the bottle
Low pressure from air in bottle pushing up
High pressure outside air pushes egg into bottle
Erlenmeyer flask with balloon• Boiling water creates
steam• Steam pushes air out of
flask• Balloon seals flask• Ice cools steam back to
water• Because flask is sealed
no air can get back in creating low pressure inside flask
• High pressure on outside pushes the balloon into the flask
Boiling water in vacuum
• Before vacuum pump is turned on the air pressure and water pressure are equal
• Vacuum pump takes away the air pressure inside the bell jar pushing down on the water
• Water continues to push up
No air pressure
Straw in water• Inside the straw the air
pressure pushing down and the water pressure pushing up are equal
• Under the straw because of surface tension, the water forms a flat surface for the air pressure to push on
• Air pressure is pushing harder then weight of the water keeping the water in the straw
• Taking your thumb away adds extra air pressure pushing down on the water
• Same explanation as suspended card
Crushed can• Boiling water creates
steam• Steam pushes air out of
can• When tipped over the water
in tank seals can• Steam cools back to water• Because can is sealed no
air can get back in creating low pressure inside can
• High pressure on outside pushes on the can crushing the can
steam
water
Steam cools
Bottles with balloons• Air has a high pressure• In the bottle without holes
the blower is trying to push against the pressure of the air in the bottle and is unsuccessful
• In the bottle with holes the air in the bottle has a place to escape so the blower is not pushing against the air pressure
Crushed egg• When trying to crush
with whole hand: large area = small pressure
• If using fingertips: small area = large pressure
• If using two hands: double the force = double the pressure
“Pirates of the Caribbean”
• Pressure of water on air and pressure of air on water are equal
• Even after we push on the glass the air will continue to push the same amount on the water so the water cannot come in the glass.
