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Chapter 15

Chapter 15. Fluids. Outline. Density Pressure Atmospheric pressure Pressure and depth Applications: barometer Water seeks its own level. Density. Definition of density , : The density of a material is defined as the mass per volume .  = M/V. SI unit: kg/m 3 .

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Chapter 15

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  1. Chapter 15 Fluids Dr. Jie Zou PHY 1151G Department of Physics

  2. Outline • Density • Pressure • Atmospheric pressure • Pressure and depth • Applications: barometer • Water seeks its own level Dr. Jie Zou PHY 1151G Department of Physics

  3. Density • Definition of density, : The density of a material is defined as the mass per volume. •  = M/V. SI unit: kg/m3. • The denser a material, the more mass it has in any given volume. • Examples of density of different materials: gold, 19,300 kg/m3; fresh water, 1000 kg/m3; helium, 0.179 kg/m3 (See Table 15-1) • Example: Find the mass of a gallon of water.(1 gallon = 3.79 L = 3.79  10-3 m3) Dr. Jie Zou PHY 1151G Department of Physics

  4. Pressure • Definition of pressure: Pressure is a measure of the amount of force per area. • P = F/A. SI unit: N/m2. • Pressure is increased if the force applied to a given area is increased, or if a given force is applied to a smaller area. This bird exerts only a small pressure on the lily pad because its weight spreads out over a large area by its long toes. The same force applied to the small area of a needle tip causes a large enough pressure to rupture the balloon. Dr. Jie Zou PHY 1151G Department of Physics

  5. Example • Find the pressure exerted on the skin of a balloon if you press with a force of 2.1 N using • (a) your finger or (b) a needle (Assume the area of your fingertip is 1.0  10-4 m2, and the area of the needle tip is 2.5  10-7 m2.) • (c) Find the minimum force necessary to pop the balloon with the needle, given that the balloon pops with a pressure of 3.0  105 N/m2. Dr. Jie Zou PHY 1151G Department of Physics

  6. Atmospheric pressure • Atmospheric pressure, Pat, is a direct result of the weight of the air above us. Pat = 1.01  105 N/m2 (about 14.7 pounds per square inch). • A Shorthand unit for N/m2 is the pascal (Pa): 1 Pa = 1 N/m2. • A common unit for atmospheric pressure in weather forecasting is the bar, defined as follows: 1 bar = 105 Pa ≈ 1 Pat. • Gauge pressure: Pg = P - Pat Dr. Jie Zou PHY 1151G Department of Physics

  7. Pressure and depth in fluids • At a depth h, the pressure P is given by: • P = Pat + gh. • : density of the fluid • If the pressure at one point is P1, the pressure P2 at a depth h below that point is the following: • P2 = P1 + gh. Pat P Dr. Jie Zou PHY 1151G Department of Physics

  8. Example • The titanic was found in 1985 lying on the bottom of the North Atlantic at a depth of 2.5 miles. What is the pressure at this depth? (The density of sea water is 1025 kg/m3 and 1 mile = 1609 m.) Dr. Jie Zou PHY 1151G Department of Physics

  9. Application: barometer • A simple barometer: A device used to measure atmospheric pressure • Atmospheric pressure Pat is related to the height of fluid in the tube by the relation: Pat = gh. Dr. Jie Zou PHY 1151G Department of Physics

  10. Water seeks its own level • Water seeks its own level. • Example 15-4: A U-shaped tube is filled mostly with water, but a small amount of vegetable oil has been added to one side. wateris 1.00 x 103 kg/m3, and oil is 9.20 x 102 kg/m3. If the depth of the oil is 5.00 cm, what is the difference in level h between the top of the oil on one side of the U and the top of the water on the other side? Dr. Jie Zou PHY 1151G Department of Physics

  11. Homework • See online assignment on www.masteringphysics.com Dr. Jie Zou PHY 1151G Department of Physics

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