Pressure

1. Stand flat on the ground for 30 seconds. Now do the same thing but on your tiptoes. Was there a difference in how it felt/difficulty? (I don't mean balance) What changed? Why do they freak out when women wear high heels on the basketball gym floor? Pressure

2. Atmospheric Pressure Does the air exert pressure on you? Water bottle demo (p. 23 Steve Spangler) babyhead_second_silent.mpg Stab the potato! http://www.youtube.com/watch?v=BfbDG7oeqC0&sns=em

3. Fluid Pressure All matter is made up of molecules - the forces that exist in fluids are caused by the mass and motion of the particles making up the fluid. Gases have mass and behave very much like liquids so “fluids” include both liquids and gases. Ex: Dry Ice vs water http://youtu.be/Jz7BgxrVmiQ As particles move, they push against the other particles and against the walls of its container, with a force dependent on the mass of the particles and the acceleration of the particle (Newton’s 2nd Law)

4. Fluid Pressure The force that the particles exert over a certain area is called pressure. Fluid pressure is exerted equally in all directions. Pressure is defined by scientists as force per unit area: Pressure = Force / Area Units: As Force is measured in newtons, and area in m2, pressure is measured in newtons per square meter (N/m2) or Pascals (Pa)

5. Atmospheric Pressure https://www.youtube.com/watch?v=xmYss_hNF4Q&feature=youtube_gdata_player Air in the atmosphere exerts a pressure of 10.13 N/cm2 at sea level. If your back has an area of 1000 cm2……. P = F/A 10.13 N/cm2 = F (1000 cm2) F = 10,130 N

6. Pascal’s Principle If there are no outside forces acting on a fluid, the pressure exerted by the fluid will be the same throughout, and will be exerted evenly in every direction If a force is exerted, then the resulting change in pressure will act evenly in all directions – Pascal’s Principle This transmission of pressure equally in all directions in a liquid is the principle behind hydraulic devices.

7. Pascal’s Principle Hydraulic devices multiply forces by using pressurized fluid acting on pistons of different sizes. Small applied force on a small piston results in a much larger force - why? The same amount of pressure acts on every square cm of surface - the surface on the larger piston is much larger, so larger force. F = P/A On a car, the larger piston attached to the brakes would slow the car with only the slight push of your foot.

8. Pascal’s Principle F2 = Pressure * area = 10 N/cm2 * 100 cm2 = 1000 N F1 = 10 N A2 = 100 cm2 A1 = 1 cm2 P1 = F / A = 1000N / 100 cm2 = 10 N/cm2 P1 = F / A = 10N / 1 cm2 = 10 N/cm2 Input Force over a small area results in a HUGE output Force over a large area, BUT pressure is exactly the same!!!!!

9. Pressure & Depth There is one force that is always present, pulling down on you - Gravitational forces increase as you near the center of the Earth So, the lower you are in a fluid, the greater the pressures!

10. Pressure & Depth Air pressure in Denver is 83% of that at sea level. Gravitational forces decrease as you move farther from the center of the Earth (climb a mountain, fly in a plane) Fewer and fewer particles of fluid above you = less pressure. Altitude sickness – Why is it harder to breathe at higher altitudes? O2 makes up 21% of the air we breathe – at 30,000 feet, that percentage is the same 21% (78% is N) Less pressure is the culprit – why? Videos of induced hypoxia

11. Atmospheric Pressure & Boiling https://www.youtube.com/watch?feature=player_embedded&v=k3sX00JIeCI#at=61

12. Pressure & Depth More and more particles of fluid above you = more pressure. Submarines crushed by the pressure Decompression Sickness (The Bends) – increasing pressure causes more than normal amounts of N gas to dissolve in your blood as you dive deeper - as you come up, pressure lessens, and the gas is released - too quickly can cause tiny bubbles that are painful & possible deadly.

13. Pressure & Depth At the deepest point in our oceans, the pressure is approx 11,031 N/cm2 - about the weight of 50 jumbo jets! Remember, for comparison, the air pressure you are comfortable with is approximately 10.13 N/cm2

14. Pressure increases with depth, not width. The pressure at 10 meters down in a swimming pool is the same as the pressure at 10 meters in a lake. Ex: Water towers and water pressure in your home Pressure & Depth

15. Fluids will always move from areas of higher pressure to areas of lower pressure. Using a straw to drink – placing the straw in your mouth results in the liquid being placed between an area of low air pressure (your lungs) and an area of higher pressure (the air pushing down on the top of the drink); the air pressure pressing on the liquid in the glass is greater than the pressure inside the straw, so the liquid is forced up. Notice, you cannot swallow until if the valve to your lungs is still open, so when that valve closes, the pressure difference is gone and liquid stops flowing while you swallow. Changes in Pressure

17. Changes in Pressure Explain: Plunger, Harbottle Where is the pressure change: How did it change? Explain what happened…The higher change ____ (where) compared to the lower change ____ (where) caused _______ (what to happen). Egg in a bottle http://www.youtube.com/watch?v=28TIyWdfxxc Spiderman suit -!!! http://www.youtube.com/watch?v=6PncjDDYKcE (alt link: http://youtu.be/6PncjDDYKcE)

18. The Gas Laws http://ed.ted.com/lessons/1207-1-a-bennet-brianh264

19. Gas Laws Remember when we added the hot water to a coke bottle, swirl it around, and then dump it out and quickly cap it. What happened? Why?

20. Gas Laws Charles’ Law: As the temperature of a gas increases, the pressure it exerts increases causing volume to increase...This is a DIRECT PROPORTION Tire pressure Hot air balloon Exploding can in a campfire

21. Gas Laws Charles’ Law: As the temperature of a gas decreases, the pressure it exerts decreases causing volume...This is a DIRECT PROPORTION As pressure decreases, temperature decreases-compressed air, iced windows of plane of Payne Stewart in 1999 Compressed air

22. Gas Laws Charles’ Law: The mathematical expression that shows this relationship between volume and temperature is: V1 = V2 T1 T2 **Temperature must be in Kelvin!

23. Gas Laws Boyle’s Law: As the volume of a gas decreases (i.e. the size of the container gets smaller), the pressure it exerts on it’s container increases…….This is an INVERSE PROPORTION Shrink the container, the gas molecules will run into one another and the walls of the container more often.

24. Gas Laws Boyle’sLaw: The mathematical expression that shows this relationship between pressure and volume is: P1V1 = P2V2

25. Gas Laws The Combined Gas Lawcombines both Charles’ and Boyle’s laws to show the relationship between pressure, volume and temperature. P1V1 = P2V2 T1 T2 **Temperature must be in Kelvin

26. Solve A cylinder that contains air at a pressure of 100 kPa has a volume of 0.75L. The pressure is increased to 300kPa. The temperature does not change. Find the new volume of air. Known: Unknown: Formula: (What does not change? Leave it out!) Solve:

27. Bernoulli’s principle: the pressure in a moving stream of fluid is less than the pressure of the fluid surrounding it……the faster a fluid moves the less pressure it exerts. Explains why flight is possible. Blowing across the piece of paper held in a book Fluids in Motion

29. Airplane wings: The shape causes the air to move more quickly over the wing than under (s = d/t….greater distance to travel in same amt of time as stream passing under wing) The faster air has less pressure than the air under the wing, so the resulting upward force helps produce the lift. Fluids in Motion

30. Fluids will always move from areas of higher pressure to areas of lower pressure. Vacuum cleaner: A fan changes the pressure inside the canister - how? The pressure inside the canister is ________ than the pressure outside. The outside air pressure forces the air, and dirt along with it, into the area of lower pressure. Changes in Pressure

32. White Boards & Demos- Explain the following Atomiser (beaker of water, straws, color paper to hold behind for evidence) Explain Bernoulli Bags demo…..Subway Sandwich Bags! How is it possible to siphon gas out of a tank using a water hose? Getting it started makes sense, but why does it continue to come out once you drop the other end into a bucket? Smoke sucked out of a car window – when standing still? Moving? Why? Small convertible car sucked underneath a semi truck – why? The spoiler on the back of a car is shaped to push down on the back of the car for more traction. Would you want the air to flow faster above or below the spoiler? Extra…..ship passing under a bridge that it will just barely clear – why would moving faster help?

33. Archimedes’ Principle Why is it possible to lift an object in water, that you may not be able to lift in normal conditions? For an object in water, the force at the bottom of the object is greater than the force at the top, causing an upward force called a buoyantforce. Thebuoyant forceon an object is equal to the weight of the fluid displaced by the object…. Archimedes’ principle.

34. Buoyancy You have two blocks, the same exact size and shape, and displace the same amount of water. When placed in the water, the gray block floats just at the surface, and the brown one sinks. Which experiences the greater buoyant force – the grey or the brown? Why? If same buoyant force, then why does the brown one sink? 1 2

35. Buoyancy Thebuoyant forcecan be greater than, less than, or equal to the weight of an object placed in the fluid. When the buoyant force is equal to the weight, an object floats or is suspended. When the maximum buoyant force possible is less than the weight, the object sinks. Submarines – https://www.youtube.com/watch?v=ZiModwp_RIk&feature=youtube_gdata_player

36. Buoyancy Why do some objects float lower than others? More weight means they will float lower, as long as the weight is not more than the maximum buoyant force pushing up. Submarines – https://www.youtube.com/watch?v=ZiModwp_RIk&feature=youtube_gdata_player

37. Compare a box filled with air, cotton balls, books, etc..... Which changes – mass or volume? Which did NOT change? What makes the mass different when they are the same volume? Could we put more cotton in the same box? How? What would be the buoyant force on each of these boxes? Buoyancy Layer liquids (p.71 SS)

38. Buoyancy The buoyant force is determined by the weight of the water displaced So, if you have two objects with the same volume, (and therefore the same buoyant force) why would one float and the other sink? DENSITY

39. Buoyancy DENSITY = how much mass occupies a certain volume. Density = mass / volume More matter packed into the same volume = more downward force When an object’s density is less than that of the fluid, then the object would weigh less than the buoyant force so it floats. When an object’s density is greater than that of the fluid, then the object weighs more than its buoyant force so it sinks.

41. Buoyancy A balloon filled with water compared to the same volume balloon filled with helium The buoyant force (upward arrow) is the same for both because it is the weight of the air being displaced. Since both balloons have the same volume, they displace the same amount of air. Water has a greater density than helium, and therefore more weight than helium. (downward arrow) Most fish have a gas-filled bladder that they can adjust to let in more or less gas = sink, rise or float. Submarines helium water

42. Buoyancy Dancing Raisins – explain Why do some coke cans float and others sink? WHICH were floaters and which were sinkers? Any correlation?

43. Why do people float higher in the Dead Sea than in a swimming pool? What is different about the Dead Sea?