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?

Applications

Air Pressure  Standard air pressure = N/cm 2 or ~ 15 lbs/in 2  Area of your back = 1000 cm 2  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?

Barometer Used to measure atmospheric pressure h

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 Water with vacuum Straw in water Crushed can Blowing up balloons in bottles Crushed egg “Pirates of the Caribbean”

A tornado produces a region of extreme low pressure. If a house is near a tornado, is it likely to explode or implode. Why? Low pressure outside and high pressure inside

Hydraulics

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 the weight of the object _______ the weight of water displaced  If an object sinks the weight of the object _______ the weight of water displaced.

Archimedes’ Principle Silly 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?

What is the buoyant force from the fluid on the object? Weight of object in air = 32.0 N Weight of object in fluid = 10.0 N Will the object float or sink?

What is the weight of the object in air? Buoyant force = 17. N Weight of object in fluid = 0 N Will the object float or sink?

What is the buoyant force on the object? Mass of displaced water = 408. g Weight of object in air = 5.2 N Will the object float or sink?

What is the weight of the object in the fluid? Buoyant force of the fluid on the object = 25. N Mass of object = 4.5 kg Will the object float or sink?

When placed in water which will displace more fluid? Buoyant force = 12. NBuoyant force = 20. N When placed in water will the objects float or sink? Mass of object = 1.22 kg Mass of object = 3.51 kg

Why do people seem to weigh less under water?

How does a submarine dive?

 Which has a better chance of floating? Why? 50g

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 Toy helicopter Finding Nemo

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.