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. Barometer
Used to measure atmospheric pressure h

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. A tornado produces a region of extreme low pressure
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

13. Hydraulics

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

15. 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
Buoyancy
Force of gravity or weight of object
Buoyancy
Force of gravity or weight of object

16. 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

17. 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?

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

19. How does a submarine dive?

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

21. 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?

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

23. 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

24. 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

25. 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

26. 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?

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

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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

35. “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.