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# Fluid mechanics

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### Transcript

• 1. FLUID MECHANICS
• 2. Game: The Foilboat Contest
• Each group will construct a foilboat out of a 6” x 6” foil. The boat should support as many marbles as it can without sinking. The boat which can hold the most number of marble wins.
• 3. Why does a raft flow on water? Why is water turbulent in the rapids and smooth in other places on a river?
• 4. Defining a Fluid
• A fluid is a nonsolid state of matter in which the atoms or molecules are free to move past each other, as in a gas or a liquid.
• Both liquids and gases are considered fluids because they can flow and change shape.
• Liquids have a definite volume; gases do not.
• 5. Density
• The concentration of matter of an object is called the density .
• ρ = mass
• volume
• 6.
• The objects with lesser density than the fluid where it is immersed will float.
• 7. Buoyant forces can keep objects afloat.
• Have you ever wondered why things feel lighter underwater than they do in air?
• 8. Buoyant Force
• The fluid exerts an upward force on objects that are partially or completely submerged in it.
• 9.
• 10.
• 11. Displaced Volume of a Liquid
• 12. Apparent Weight Supports Buoyant Force Idea
• 13. Archimedes’ Principle determines the amount of buoyancy.
• 14. Archimedes’ Principle
• Any object completely or partially submerged in a fluid experiences an upward buoyant force equal in magnitude to the weight of the fluid displaced by the object.
• F B = F g (displaced fluid) = m f g
• magnitude of buoyant force = weight of fluid displaced
• 15. Buoyant Force on Floating Objects
• 16. The density of an object determines the depth of submersion.
• 17. Buoyancy can be changed by changing average density.
• 18. Conceptual Challenge
• 19. Astronauts sometimes train underwater to simulate conditions in space. Explain why.
• 20. Explain why balloonists use helium instead of pure oxygen in balloons.
• 21. Sample Problem
• A bargain hunter purchases a “gold” crown at a flea market. After she gets home, she hangs the crown from a scale and finds its weight to be 7.84 N. She then weighs the crown while it is immersed in water, and the scale reads 6.86 N. Is the crown made of pure gold? Explain.
• 22.
• 23. Challenge: Eggsqueeze
• Break the egg by simply squeezing it. Make sure you use only one hand and wrap your fingers around the egg. You must not wear rings.
• 24. Challenge: Floating Clips
• Make a paper clip float in water.
• 25. Why do deep sea explorers have to wear diving suits?
• 26. Pressure
• It is the magnitude of the force on a surface per unit area.
• Pressure = Force
• Area
• 27. Unit
• 1 Pascal (Pa) = 1 N / m 2
• 1atm = 1.013 x 10 5 Pa
• (atmospheric pressure at sea level)
• 28. Conversion to other units
• 1 bar = 100 000 Pa
• 1 torr = 133.32 Pa
• 1 psi = 6.894 x 10 3 Pa
• 29.
• 30. Sample Problem
• A force of 90 N is applied on a square surface with a length of 0.05 m. How much pressure is applied on the surface?
• If the same amount of force is applied in a smaller square with a side equal to 0.025 m, how much pressure does it experience?
• 31. The Science of Toothpaste
• 32. Applied pressure is transmitted equally throughout a fluid.
• 33. Pascal’s Principle
• The pressure applied to a fluid in a closed container is transmitted equally to every point of the fluid and to the walls of the container.
• 34.
• 35.
• P 1 = P 2
• F 1 = F 2
• A 1 A 2
• 36. Sample Problem
• The small piston of a hydraulic lift has an area of 0.20 m2. A car weighing 1.60 x 104 N sits on a rack mounted on the large piston. The large piston has an area of 0.90 m2. How large a force must be applied to a small piston to support the car?
• 37. Pressure varies with depth.
• 38. Water pressure increases with depth because the water at a given depth must support the weight of the water above it.
• 39. Gauge Pressure
• It is the pressure due to the weight of the column of liquid on top of the object.
• P = ρ gh
• 40. Absolute Pressure
• P = Po + ρ gh
• Absolute pressure = atmospheric pressure + gauge pressure
• 41. Atmospheric Pressure
• It is the pressure from above.
• Kinetic Theory of Gases can describe the origin of gas pressure.
• 42.
• 43.
• 44. Sample Problem
• Calculate the absolute pressure at an ocean depth of 1.00 x 10 3 m. Assume that the density of the water is 1.025 x 10 3 kg/m 3 and that Po = 1.01 x 10 5 Pa.
• 45. Conceptual Challenge
• 46. A woman wearing snowshoes stands safely in the snow. If she removes her snow shoes, she quickly begins to sink. Explain what happens in terms of Force and Pressure.
• 47.
• 48.