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9-1. Density and Pressure. Density. The density of a substance of uniform composition is defined as its mass per unit volume: SI unit: kg/m 3 (SI) Often see g/cm 3 (cgs) 1 g/cm 3 = 1000 kg/m 3. Section 9.2. Density, cont.

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9-1

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  1. 9-1 Density and Pressure

  2. Density • The density of a substance of uniform composition is defined as its mass per unit volume: • SI unit: kg/m3 (SI) • Often see g/cm3 (cgs) • 1 g/cm3 = 1000 kg/m3 Section 9.2

  3. Density, cont. • The densities of most liquids and solids vary slightly with changes in temperature and pressure • Densities of gases vary greatly with changes in temperature and pressure • The higher normal densities of solids and liquids compared to gases imply that the average spacing between molecules in a gas is about 10 times greater than the solid or liquid Section 9.2

  4. Specific Gravity • The specific gravity of a substance is the ratio of its density to the density of water at 4° C • The density of water at 4° C is 1000 kg/m3 • Specific gravity is a dimensionless quantity Section 9.2

  5. Pressure • The force exerted by a fluid on a submerged object at any point is perpendicular to the surface of the object • The average pressure P is the force divided by the area Section 9.2

  6. Measuring Pressure • The spring is calibrated by a known force • The force the fluid exerts on the piston is then measured

  7. Example – The Water Bed.A water bed is 2.00 m on each side and 30.0 cm deep. What is its weight and what pressure does it exert on the floor?

  8. Variation of Pressure with Depth • If a fluid is at rest in a container, all portions of the fluid must be in static equilibrium • All points at the same depth must be at the same pressure • Otherwise, the fluid would not be in equilibrium • The fluid would flow from the higher pressure region to the lower pressure region Section 9.4

  9. Pressure and Depth • Examine the darker region, assumed to be a fluid • It has a cross-sectional area A • Extends to a depth h below the surface • Three external forces act on the region Section 9.4

  10. Pressure and Depth equation • Po is normal atmospheric pressure • 1.013 x 105 Pa = 14.7 lb/in.2 • The pressure does not depend upon the shape of the container Section 9.4

  11. Pascal’s Principle • A change in pressure applied to an enclosed fluid is transmitted undiminished to every point of the fluid and to the walls of the container. • First recognized by Blaise Pascal, a French scientist (1623 – 1662) Section 9.4

  12. Pascal’s Principle, cont • The hydraulic press is an important application of Pascal’s Principle • Also used in hydraulic brakes, forklifts, car lifts, etc. Section 9.4

  13. In a car lift in a service station, compressed air exerts a force on a small piston with a radius of 5.00 cm. The second piston attached to the lift has a radius of 15.0 cm. What force must the air produce to lift a 13,300 N car? What is the air pressure required? Show that the work done by the pistons is the same.

  14. In a car lift in a service station, compressed air exerts a force on a small piston with a radius of 5.00 cm. The second piston attached to the lift has a radius of 15.0 cm. What force must the air produce to lift a 13,300 N car? What is the air pressure required? Show that the work done by the pistons is the same.

  15. Pressure Measurements:Manometer • One end of the U-shaped tube is open to the atmosphere • The other end is connected to the pressure to be measured • If P in the system is greater than atmospheric pressure, h is positive • If less, then h is negative Section 9.5

  16. Absolute vs. Gauge Pressure • The pressure P is called the absolute pressure • Remember, P = Po + ρgh • P – Po = ρgh is the gauge pressure Section 9.5

  17. Pressure Measurements: Barometer • Invented by Torricelli (1608 – 1647) • A long closed tube is filled with mercury and inverted in a dish of mercury • Measures atmospheric pressure as ρgh Section 9.5

  18. Pressure Values in Various Units • One atmosphere of pressure is defined as the pressure equivalent to a column of mercury exactly 0.76 m tall at 0o C where g=9.806 m/s2 • One atmosphere (1 atm) = • 76.0 cm of mercury • 1.013 x 105 Pa • 14.7 lb/in2 Section 9.5

  19. Blood Pressure • Blood pressure is measured with a special type of manometer called a sphygmomano-meter • Pressure is measured in mm of mercury Section 9.5

  20. Archimedes • 287 – 212 BC • Greek mathematician, physicist, and engineer • Buoyant force • Inventor Section 9.6

  21. Archimedes' Principle • Any object completely or partially submerged in a fluid is buoyed up by a force whose magnitude is equal to the weight of the fluid displaced by the object Section 9.6

  22. Buoyant Force • The upward force is called the buoyant force • The physical cause of the buoyant force is the pressure difference between the top and the bottom of the object Section 9.6

  23. Buoyant Force, cont. • The magnitude of the buoyant force always equals the weight of the displaced fluid • The buoyant force is the same for a totally submerged object of any size, shape, or density Section 9.6

  24. Buoyant Force, final • The buoyant force is exerted by the fluid • Whether an object sinks or floats depends on the relationship between the buoyant force and the weight Section 9.6

  25. Archimedes’ Principle:Totally Submerged Object • The upward buoyant force is B = ρfluidVobjg • The downward gravitational force is W = mg = ρobjVobjg • The net force is B – W = (ρfluid-ρobj)Vobjg Section 9.6

  26. Totally Submerged Object • The object is less dense than the fluid • The object experiences a net upward force Section 9.6

  27. Totally Submerged Object, 2 • The object is more dense than the fluid • The net force is downward • The object accelerates downward Section 9.6

  28. Archimedes’ Principle:Floating Object • The object is in static equilibrium • The upward buoyant force is balanced by the downward force of gravity • Volume of the fluid displaced corresponds to the volume of the object beneath the fluid level Section 9.6

  29. Archimedes’ Principle:Floating Object, cont • The forces balance • Neglects the buoyant force of the air Section 9.6

  30. You purchase a “gold” crown at Crowns-Я-Us. You hang it from a scale and its weight is 7.84 N. You then weigh the crown while in a bucket of water, and the scale reads 6.86 N. Is the crown pure gold?

  31. You purchase a “gold” crown at Crowns-Я-Us. You hang it from a scale and its weight is 7.84 N. You then weigh the crown while in a bucket of water, and the scale reads 6.86 N. Is the crown pure gold?

  32. A raft is constructed of wood having a density of 600 kg/m3. Its surface area is 5.70 m2 and its volume is 0.60 m3. When placed in fresh water, to what depth is the bottom of the raft submerged?

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