advertisement

13 Liquids • Demos: • Surface tension (pepper, soap) • Homework: • RQ: 2, 4, 7, 10, 12, 14, 21. • Problems: 2, 10. 1 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. 2 Depth and Fluid Pressure • pressure = weight density x depth • depth only (not area), pressure equal at bottom of all shapes below 3 Buoyancy and Archimedes’ Principle 4 sink or float? • object more dense than fluid sinks • object less dense than fluid floats • object same density as fluid is neutral 5 Water Density & Volumes • • • • “L” = liter (about 33oz) 1mL = 0.001L = 1cc 1mL water = 1gram water density = 1gram/mL 6 Example Volumes • An 800 gram object displaces 400mL (submerged). What is its density? • Density = mass/volume • = 800grams/400mL • = 800grams/400cm3 • = 2g/cm3 7 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 8 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 9 Pascal’s Principle • pressure increases on an enclosed fluid are transmitted undiminished to entire fluid. • Hydraulic Application: • output force = (area ratio)x(input force) 10 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 11 Videos • • • • • Metal boats Effect of Screen Soap in water, floating Soap in milk, food coloring Water, pepper 12 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) 13 Summary • pressure = force/area • pressure ~ depth • buoyant force equals weight of displaced fluid • pressure changes are distributed throughout enclosed fluids 14 Summary 2 • surface tension phenomena • surface area/volume ratio low for sphere, high for sheet 15 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 16 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? 17