1. Fluids Honors Physics

2. Liquids In a liquid, molecules flow freely from position to position by sliding over each other Have definite volume Do not have definite shape – conform to their container

3. Density Mass Density ρ = m/V Units – kg/m 3 Common densities Air – 1.29 kg/m 3 Fresh water – 1.00 x 10 3 kg/m 3 Ice - 0.917 x10 3 kg/m 3

4. Densities of Common Substances

5. Buoyancy The apparent loss of weight of an object that is submerged The water exerts an upward force that is opposite the direction of gravity called the buoyant force.

6. Submerged An object placed in water will displace, or push aside, some of the water The volume of water displaced, is equal to the volume of the object This method can be used to easily determine the volume of irregularly shaped objects

8. Archimedes’ Principle An immersed object is buoyed up by a force equal to the weight of the fluid it displaces. This principle is true for all fluids. This means that the apparent weight of an immersed object is its weight in air minus the weight of the water it displaces For floating objects F B = F g (object)

10. Examples A brick with a mass of 2kg weighs 19.6N If it displaces 1L of water, what is the buoyant force exerted on the brick? Buoyant force = weight of water displaced 1L displaced = 9.8N Buoyant force = 9.8N

11. Sink or Float? If the buoyant force acting on an object is greater than its weight force, the object will float A submerged objects’ volume, not mass determines buoyant force 3 Rules An object more dense than the fluid it is immersed in will sink An object less dense than the fluid it is immersed in will float An object with equal density to the fluid will neither sink nor float.

13. Density & Buoyant Force The buoyant force and apparent weight of an object depends on density

14. Sample Problem 9A 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 make of pure gold? Explain.

15. Floatation Why is it possible for a brick of iron to sink, but an equal mass of iron shaped into a hull will float? When the iron is shaped, it takes up more space (volume) Principle of Flotation – A floating object displaces a weight of fluid equal to its own weight

16. Liquid Pressure Pressure for solids is determined by the equation P=F/A In this equation, the force is simply the weight of the object. The same principle can be used for liquids

17. Pascal’s Principle 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. F 2 = A 2 F 1 A 1

18. Sample Problem 9B The small piston of a hydraulic life has an area of 0.20 m 2. A car weighing 1.20 x 10 4 N sits on a rack mounted on the large piston. The large piston has an area of 0.90 m 2. How large a force must be applied to the small piston to support the car?

19. Pressure More dense liquids will produce more force and, therefore, more pressure. The higher the column of liquid the more pressure also. For liquids, Pressure = density x g x depth AKA Gauge Pressure = ρgh Total pressure = density x g x depth + atmospheric pressure P = P O + ρgh

20. Examples Is there more water pressure at 3m or at 9m of depth? Calculate the pressure exerted by a column of water 10m deep. 9m =98000 Pa

21. Sample Problem 9C 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 P O =1.01 x 10 5 Pa.

22. Pascal’s Principle Changes in pressure at any point in an enclosed fluid at rest are transmitted undiminished to all points in the fluid and act in all directions. Hydraulic systems operate using this principle.

23. Gasses Have neither definite volume nor shape The atmosphere is a good example of a gas. In the atmosphere, the molecules are energized by sunlight and kept in continual motion

24. Atmosphere The density of the atmosphere decreases with altitude Most of the Earth’s atmosphere is located close to the planets surface.

25. Atmospheric Pressure The atmosphere all around us exerts pressure just as if we were submersed in a liquid At sea level, air has a density of about 1.2 kg per cubic meter A column of air, of 1 sq. meter that extends up through the atmosphere weighs about 100,000 N The avg atmospheric pressure a sea level is 101.3 kPa

26. Measuring Pressure A barometer is used to measure atmospheric pressure Air pressure forces mercury up the glass tube, to display the pressure This process is similar to that of drinking out of a straw

27. Boyle’s Law For a gas, the product of the pressure and the volume remain constant as long as the temperature does not change. P 1 V 1 = P 2 V 2

28. Examples If you squeeze a balloon to 1/3 its original volume, what happens to the pressure inside? 3x A swimmer dives down, until the pressure is twice the pressure at the waters surface. By how much does the air in the divers lungs contract? 2x

29. Charles’ Law The volume of a definite quantity of a gas varies directly with the temperature, provided the pressure remains constant. V 1 T 2 = V 2 T 1

30. Combined Gas Law When Boyle’s and Charles’ laws are combined the equation looks like this. P 1 V 1 T 2 = P 2 V 2 T 1

31. Sample Problem 9E Pure helium gas is contained in a leakproof cylinder containing a movable piston. The initial volume pressure and temperature of the gas are 15 L, 2.0 atm and 310 K, respectively. If the gas is rapidly compressed to 12 L and the pressure increased to 3.5 atm, find the final temperature of the gas.

32. Ideal Gas Law Compares volume, pressure, and temperature of a gas PV = Nk B T P = pressure, V = volume, N = # of mols of gas particles, k B = Boltzman’s Constant (1.38x10 -23 J/K), T = temperature

33. Fluid Flow Smooth flow is said to be laminar flow Particles all follow along a smooth path Streamline path Streamlines never cross Irregular flow is said to be turbulent Irregular motion produced are called eddies

34. Continuity Continuity says that the mass of and ideal fluid flowing into a pipe must equal to mass flowing out of the pipe. Or m 1 = m 2 Because the mass flowing is determined by the cross-sectional area of the pipe and how fast it flows, we can also say A 1 v 1 = A 2 v 2

35. Bernoulli’s Principle Pressure in a fluid decreases as the fluid’s velocity increases. Bernoulli’s Principle can be seen in birds in flight and airplanes Pressure above the wing is less than pressure below the wing, creating lift

36. Bernoulli’s Equation This is an expression of conservation of energy in a fluid. P + ½ρv 2 + ρgh = constant Pressure + kinetic energy per unit volume + gravitational potential energy per unit volume = constant along a given streamline

37. Sample Problem 9D A water tank has a spigot near its bottom. If the top is open to the atmosphere, determine the speed at which the water leaves the spigot when the water level is 0.500m above the spigot. We’ll use (P + ½ρv 2 + ρgh) 1 = (P + ½ρv 2 + ρgh) 2

38. we assume the water level is dropping slowly, so v 2, at the top, = 0 Also, since both ends are open to the atmosphere P 1 = P 2 That simplifies the equation to P + ½ρv 1 2 + ρgh 1 = P + ρgh 2 and subtract P ½ρv 1 2 + ρgh 1 = ρgh 2 ρ is the same throughout, so ½v 1 2 + gh 1 = gh 2 solve for v v = √(2g(h 2 -h 1 ))plug & chug v = √(2(9.8 m/s 2 )(.5m)) v = 3.13 m/s

39. Pg. 344: 17, 18, 23, 25, 29, 36, 39, 44, 47, 48 Test on Fluids: Thursday