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13 Liquids

13 Liquids. Demos: Surface tension (pepper, soap) Homework: RQ: 2, 4, 7, 10, 12, 14, 21. Problems: 2, 10. Pressure. pressure = force/area Example: 500N applied to 0.10m 2 . Pressure = 500/0.10 = 5,000N/m 2 . Example: atmospheric pressure is 100,000 N/m 2. Depth and Fluid Pressure.

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13 Liquids

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  1. 13 Liquids • Demos: • Surface tension (pepper, soap) • Homework: • RQ: 2, 4, 7, 10, 12, 14, 21. • Problems: 2, 10.

  2. Pressure • pressure = force/area • Example: 500N applied to 0.10m2. Pressure = 500/0.10 = 5,000N/m2. • Example: atmospheric pressure is 100,000 N/m2.

  3. Depth and Fluid Pressure • pressure = weight density x depth • depth only (not area), pressure equal at bottom of all shapes below

  4. Buoyancy and Archimedes’ Principle

  5. sink or float? • object more dense than fluid sinks • object less dense than fluid floats • object same density as fluid is neutral

  6. Water Density & Volumes • “L” = liter (about 33oz) • 1mL = 0.001L = 1cc • 1mL water = 1gram • water density = 1gram/mL

  7. Example Volumes • An 800 gram object displaces 400mL (submerged). What is its density? • Density = mass/volume • = 800grams/400mL • = 800grams/400cm3 • = 2g/cm3

  8. Example 1 Arch. • A 1kg object displaces 200mL of water (submerged). Does it sink or float? • Density = 1000g/200mL • = 5g/mL = 5g/cm3 • Density > 1 Sinks

  9. Example 2 Arch. • A 1kg object displaces 1200mL of water (submerged). Does it sink or float? • Density = 1000g/1200mL • = 0.83g/mL = 0.83g/cm3 • Density < 1g/cm3 Floats

  10. Pascal’s Principle • pressure increases on an enclosed fluid are transmitted undiminished to entire fluid. • Hydraulic Application: • output force = (area ratio)x(input force)

  11. surface tension • cohesive nature of surface of a fluid, e.g. waterbug, pin floating on water • hot water has less surface tension, as does soapy water

  12. Videos • Metal boats • Effect of Screen • Soap in water, floating • Soap in milk, food coloring • Water, pepper

  13. surface area • sphere has smallest ratio of surface area/volume of any shape • surface tension causes fluids to form ‘drops’ (water has more than 3 times the surface tension of alcohol)

  14. Summary • pressure = force/area • pressure ~ depth • buoyant force equals weight of displaced fluid • pressure changes are distributed throughout enclosed fluids

  15. Summary 2 • surface tension phenomena • surface area/volume ratio low for sphere, high for sheet

  16. Ex: Sphere 12m3. • area/volume = 4pR2/(4/3)pR3 = 3/R. • Example: volume = 12 cubic meters. • R = 1.42 meters • area/volume = 3/1.42meter = 2.11/meter

  17. 12m3 rectangular plates • 1) height = 3m, width = 2m, depth = 2m • 2) height = 0.75m, width = 4m, depth = 4m • 3) height = 0.12m, width = 10m, depth = 10m • calculate area/volume ratio for these three objects. • How do these ratios compare to the sphere?

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