Forces in Fluids


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Forces in Fluids

  1. 1. Book M – Chapter 3
  2. 2. Warmup One afternoon, you go outside to find your younger sister standing by her bike with a nail in her hand. The bike has a flat tire. She wants to know why the air came out of the tire when she pulled the nail out. Write a few sentences to explain why air rushes out of a hole in a tire.
  3. 3. Taking Flight p.65
  4. 4. Section 1: Fluids and Pressure By the end of this section, you should be able to:  Describe how fluids exert pressure.  Analyze how atmospheric pressure varies with depth.  Explain how depth and density affect water pressure.  Give examples of fluids flowing from high to low pressure.
  5. 5. Fluids Exert Pressure  A fluid is any material that can flow and that takes the shape of its container. Fluids include liquids and gases.  All fluids exert pressure, which is the amount of force exerted per unit area of a surface.
  6. 6. Fluids Exert Pressure  In a tire the force of the air particles hitting the inner surface of the tire creates pressure, which keeps the tire inflated.
  7. 7. Calculating Pressure 
  8. 8. Fluids Exert Pressure  When you blow a soap bubble, why does it get rounder instead of just longer?  Soap bubbles get rounder as they get bigger because fluids exert pressure evenly in all directions.  Since air is a fluid, adding air to an air bubble causes it to expand in all directions at once.
  9. 9. Atmospheric Pressure  The atmosphere is the thin layer of nitrogen, oxygen, and other gases that surrounds Earth.  Atmospheric pressure is the pressure caused by the weight of the atmosphere.  Atmospheric pressure is exerted on everything on Earth, including you.
  10. 10. Atmospheric Pressure  The air inside a balloon exerts pressure that keeps the balloon inflated against atmospheric pressure.
  11. 11. Atmospheric Pressure  The atmosphere stretches about 150 km above the Earth’s surface, but about 80% of the atmosphere’s gases are found within 10 km.  At the top of the atmosphere, pressure is almost nonexistent.  As you travel through the atmosphere, atmospheric pressure changes. The further down through the atmosphere you go, the greater the pressure is.
  12. 12. Atmospheric Pressure  If you travel to higher or lower points in the atmosphere, the fluids in your body have to adjust to maintain equal pressure.  You may have experienced this adjustment is your ears have “popped” when you were in a plane taking off or in a car traveling down a steep mountain road.
  13. 13. Water Pressure  Water is a fluid. So, it exerts pressure like the atmosphere does.  Like atmospheric pressure, water pressure depends on depth.
  14. 14. Water Pressure  Because water is more dense than air, a certain volume of water has more mass, and weighs more, than the same volume of air.  Water exerts more pressure than air.
  15. 15. Pressure Differences and Fluid Flow  Just by drinking through a straw you can observe an important property of fluids: Fluids flow from areas of high pressure to areas of low pressure.  Exhaling causes fluids to flow from high to low pressure.
  16. 16. Pressure Differences and Fluid Flow  The air pressure inside a tornado is very low. Because the air pressure outside of the tornado is higher than the pressure inside, air rushes into the tornado.  The rushing air causes the tornado to be like a giant vacuum cleaner.
  17. 17. How Do Droppers Work? Experiment with the dropper and water and then write a paragraph talking about how the dropper works. Make sure to talk about why water goes up into the dropper and why the water can be forced out.
  18. 18. Section 1 Review p.71 #2-8, 10
  19. 19. Warmup Identify which of the following objects will float in water: a rock, an orange, a screw, a quarter, a candle, a plastic-foam “peanut,” and a chalkboard eraser. Write a hypothesis about why an aircraft carrier, which weighs thousands of tons, does not sink.
  20. 20. Section 2: Buoyant Force By the end of this section, you should be able to:  Explain the relationship between fluid pressure and buoyant force.  Predict whether an object will float or sink in a fluid.  Analyze the role of density in an object’s ability to float.  Explain how the overall density of an object can be changed.
  21. 21. Buoyant Force and Fluid Pressure  Buoyant force is the upward force that keeps an object immersed in or floating on a liquid.  Archimedes’ principle states that the buoyant force on an object is an upward force equal to the weight of the fluid that the object takes the place of, or displaces.
  22. 22. Buoyant Force and Fluid Pressure  There is more pressure at the bottom of an object because pressure increases with depth. This results in an upward buoyant force on the object.
  23. 23. Weight vs. Buoyant Force  An object in a fluid will sink if its weight is greater than the buoyant force.  An object will float only when the buoyant force on the object is equal to the object’s weight.  When the buoyant force on an object is greater than the object’s weight, the object is buoyed up (pushed up) in water.
  24. 24. Weight vs. Buoyant Force  Will an object sink or float? That depends on the whether the buoyant force is less than or equal to the object’s weight.
  25. 25. Floating, Sinking, and Density  Ice floats on water because it is less dense than water. Ice, like most substances, is more dense than air. So, ice does not float in air.  One substance that is less dense than air is helium gas. A given volume of helium displaces an equal volume of air that is much heavier than itself. So, helium floats in air.
  26. 26. Ship Shape Quick Lab p.76
  27. 27. Changing Overall Density  The secret of how a ship floats is in the shape of the ship. Ships made of steel float because their overall density is less than the density of water.  A ship made out of steel, which is almost 8 times denser than water, is able to float in water.
  28. 28. Changing Overall Density  A submarine is a special kind of ship that can travel both on the surface of the water and underwater.  Submarines have ballast tanks that can be opened to allow sea water to flow in.  As water is added, the submarine’s mass increases, but its volume stays the same.
  29. 29. Changing Overall Density  Like a submarine, some fish adjust their overall density to stay at a certain depth in the water.  Most bony fishes have an organ called a swim bladder which helps them change volume.
  30. 30. Section 2 Review p.77 #2-7
  31. 31. Warmup You have been asked to design two kites. One kite will be flown in areas where there is almost always a good breeze. The other kite will be flown in areas with very little wind. What differences in design and materials are there between your two kites?
  32. 32. Section 3: Fluids in Motion By the end of this section, you should be able to:  Describe the relationship between pressure and fluid speed.  Analyze the roles of lift, thrust, and wing size in flight.  Explain Pascal’s principle.  Describe drag, and explain how it affects lift.
  33. 33. Magic Water – Straw Demo
  34. 34. What’s Going On?  Hold 2 sheets of paper so that the edges are in front of your face about 2 inches apart. The flat faces of the paper should be parallel to each other.  Blow as hard as you can between the two sheets of paper.  Write down your observations and what you think is happening.
  35. 35. Science in the Sink
  36. 36. Fluid, Speed, and Pressure  Bernoulli’s principle states that as the speed of a moving fluid increases, the fluid’s pressure decreases.  A table-tennis ball is attached to a string and swung into a stream of water, where it is held. Because the water is moving faster than air, the ball is pushed by the higher pressure of the air into an area of reduced pressure - the water stream.
  37. 37. Ping Pong Balls Defying Gravity
  38. 38. Factors that Affect Flight  Thrust is the forward force produced by a plane’s engine.  Lift is the upward force on the wing as it moves through the air.  Smaller wings keep a plane’s weight low, which also helps it move faster.  A small bird must flap its small wings at a fast pace to stay in the air, but a large bird flaps less.
  39. 39. Drag and Motion in Fluids  Drag is the force that opposes or restricts motion in a fluid. It is a force that is parallel to the velocity of the flow.  Drag is usually caused by an irregular flow of air, known as turbulence.  Lift is often reduced when turbulence causes drag.
  40. 40. Pascal’s Principle  Pascal’s principle states that a change in pressure at any point in an enclosed fluid will be transmitted equally to all parts of that fluid.  Hydraulic devices use Pascal’s principle to move or lift objects. Liquids are used in hydraulic devices because liquids cannot be easily compressed into a smaller space.
  41. 41. Pascal’s Principle  Because of Pascal’s principle, the touch of a foot can stop tons of moving metal.
  42. 42. Section 3 Review p.83 #3-9