Chapter 13

1. Chapter 13 Forces in Fluids

2. 13.1 Fluid Pressure • Pressure • Pressure- the result of a force distributed over an area • The greater the surface area of an object, the less pressure the object will exert • To calculate pressure, divide the force by the area over which the force acts • Pressure = Force/Area

3. 13.1 Fluid Pressure • Pressure = Force/Area • Force needs to be in Newtons • Area needs to be in m2 • If these are not the units you are given, you will need to convert • The resulting unit for pressure will be a pascal (Pa) • SI unit for pressure • One pascal = N/m2

4. 13.1 Fluid Pressure • Sample problem • Pressure = Force/Area • P = • F = 2700 N • A = 1.5 m2 • P = 2700/1.5 = 1800 Pa = 1.8 kPa

5. 13.1 Fluid Pressure • Pascal • The pascal is named for Blaise Pascal • French scientist • 1000 Pa = 1 kPa

6. 13.1 Fluid Pressure • Sample Problems: P = F/A • Try these on your own (don’t forget to convert if you do not have the correct units!) 1) P = 314 Pa 2) P = 3) P = 2734 kPa F = F = 2070 N F = 804 N A = 4.2 m2 A = 71 m2 A =

7. 13.1 Fluid Pressure • Pressure in Fluids • Fluid- a substance that assumes the shape of its container • Liquids and gases are fluids • Water pressure increases as depth increases. The pressure in a fluid at any given depth is constant, and is exerted equally in all directions • For a fluid that is not moving, depth and the type of fluid are the two factors that determine the pressure the fluid exerts

8. 13.1 Fluid Pressure • Pressure in Fluids • If you were to compare the amount of pressure at 25 cm in a bathtub filled with water and in a lake, what do you think your results would be? • Tub higher? Lake higher? Same? Same!

9. 13.1 Fluid Pressure • Air Pressure and the Atmosphere • The weight of Earth’s atmosphere exerts a pressure of about 101 kPa at sea level • Air pressure increases with the depth (altitude) of the atmosphere • Air pressure decreases as the altitude above sea level increases

10. 13.2 Forces and Pressure in Fluids • Transmitting Pressure in a Fluid • Remember: 1) fluids exert pressure equally in all directions at a given depth and 2) the amount of pressure exerted by a fluid depends on the type of fluid and its depth • Imagine a 2 L bottle completely filled with water and its cap tightly screwed on. The bottle is sitting upright on a table. Visualize and describe how the pressure forces act against the inside of the bottle.

11. 13.2 Forces and Pressure in Fluids • Pascal’s Principle • According to Pascal’s principle, a change in pressure at any point in a fluid is transmitted and unchanged in all directions throughout the fluid

12. 13.2 Forces and Pressure in Fluids • Hydraulic Systems • Hydraulics is the science of applying Pascal’s principle • Hydraulic System- a device that uses pressurized fluid acting on pistons of different sizes to change a force • In a hydraulic lift system, an increased output force is produced because a constant fluid pressure is exerted on the larger area of the output piston • Ex: if the large piston has 12 times the area of the small piston, then the output force is 12 times greater than the input force • Why? F = P(A)--pressure is the same on each piston, the difference in forces is directly related to the difference in areas

13. 13.2 Forces and Pressure in Fluids • Bernoulli’s Principle • Named after the Swiss scientist Daniel Bernoulli (1700-1782) • According to Bernoulli’s principle, as the speed of a fluid increases, the pressure within the fluid decreases

14. 13.2 Forces and Pressure in Fluids • Bernoulli’s Principle • Wings and Lift • The air traveling over the top of an airplane wing moves faster than the air below the wing • This faster moving air above the wing causes the air pressure above the wing to decrease (a low pressure area) • The pressure difference between the top and the bottom of the wing creates an upward force known as lift • Lift- upward force

15. 13.2 Forces and Pressure in Fluids • Bernoulli’s Principle • Spray Bottles • See the diagram on page 397 in your book

16. 13.3 Buoyancy • Buoyant Force • Buoyancy- the ability of a fluid to exert an upward force on an object placed in it • Buoyancy results in the apparent loss of weight of an object in a fluid • Buoyant Force- an upward force acting on an object in a fluid, which acts in the opposite direction of gravity

17. 13.3 Buoyancy • Archimedes’ Principle • States that the buoyant force on an object is equal to the weight of the fluid displaced by the object • Archimedes was an ancient Greek mathematician who died in 212 BC

18. 13.3 Buoyancy • Density and Buoyancy • Closely related • If an object is less dense than the fluid it is in, it will float. If the object is more dense than the fluid it is in, it will sink

19. 13.3 Buoyancy • Density and Buoyancy • Two forces act on every object in a fluid • Weight and Buoyant Force • The force of gravity, equal to the object’s weight acts downward on the object • The buoyant force, equal to the weight of the volume of displaced fluid, acts upward on the object • When the buoyant force is equal to the weight, an object floats or is suspended. When the buoyant force is less than the weight, the object sinks

20. 13.3 Buoyancy • Density and Buoyancy • Suspended • An object that has the same density as the fluid it is submerged in will be suspended (will float at any level) in the fluid • The buoyant force acting on a suspended object exactly equals the object’s weight • Submarines and some fish are able to suspend themselves in water-->partly by adjusting their density

21. 13.3 Buoyancy • Density and Buoyancy • Sinking • An object that has a weight greater than the buoyant force working on it will sink

22. 13.3 Buoyancy • Density and Buoyancy • Floating • An object that has a weight less than the buoyant force acting on it will float • An object that displaces a large amount of fluid, creating a large buoyant force (so that it counteracts its weight) will float • Objects will float more easily in dense fluids • Ex: you will float better in salt water compared to fresh water