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Pressure

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?.

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Pressure

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  1. 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?

  2. Using the same force which has more pressure, finger or palm?

  3. Applications

  4. How would you rescue an ice skater that fell through the ice?

  5. h Barometer • Used to measure atmospheric pressure

  6. 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

  7. What happens to air pressure at higher altitudes? Why?

  8. What would happen if a bag of chips sealed at sea level were taken to the top of a mountain?

  9. What would happen if a bag of chips sealed at sea level were taken to the top of a mountain?

  10. Suction Straw • “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

  11. 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”

  12. What happens to pressure in a liquid as the depth is increased?

  13. 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 Buoyancy Force of gravity or weight of object Buoyancy Force of gravity or weight of object

  14. 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

  15. Archimedes’ Principle Silly putty in a ball sunk Silly putty flattened floated Weight of silly putty > Weight of displaced water Weight of silly putty = Weight of displaced water How much water is displaced this time: more, less or the same as the first time?

  16. Why do people seem to weigh less under water?

  17. How does a submarine dive?

  18. 50g 50g • Which displaces more water? • Which has a better chance of floating? Why?

  19. 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?

  20. Bernoulli’s principle • The pressure in a moving stream of fluid is less than the pressure in the surrounding fluid

  21. 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

  22. 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 Airplane wing

  23. 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

  24. 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?

  25. Suspended Card • Air pressure is more than the weight of the water enabling the card to hold the water up in the cup

  26. Egg in Bottle High pressure outside air pushes egg into 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

  27. 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

  28. 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

  29. 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

  30. 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

  31. 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

  32. 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

  33. “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.

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