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 = ) P = 2734 kPa
F = F = 2070 N F = 804 N
A = 4.2 m2 A = 71 m 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 ( )
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