1. Chapter 7
Forces in Fluids

2. 7-1 Fluids and Pressure Pressure The force pushing on a surface
Calculating
Equal to the force exerted on a surface divided by the total area over which the force is exerted
Pressure=Force/Area
P=F/A

3. 7-1 Fluids and Pressure Pressure Calculating
Force is measured in newtons (N)
Remember a N is kg x m/s2
Take mass times 9.8 m/s2 (the force of gravity)
Area is measured in square meters (m2)
Pressure is measure in pascals (Pa)
1 N/m2 = 1 Pa

4. 7-1 Fluids and Pressure
A force of 29 N is on an area of 15 m2. What is the pressure?
P = F / A
P = 29 N / 15 m2
P = 1.93 N/m2
P = 1.93 Pa

5. 7-1 Fluids and Pressure
A force of N is on an area of .046 m2. What is the pressure?
P = F / A
P = N / .046 m2
P = N/m2
P = Pa

6. 7-1 Fluids and Pressure
A force of 296 N is placed on a table that is 2 m wide and 4 m long. What is the pressure on the ground?
P = F / A A = L * W
A = 2 m * 4 m
A = 8 m2
P = 296 N / 8 m2
P = 37 N/m2
P = 37 Pa

7. 7-1 Fluids and Pressure
F = 58 kg * 9.8 m/s2
F = kg * m/s2
F = N
A = 5 m * 7 m
A = 35 m2
P = N / 35 m2
P = N/m2
P = Pa
A person that has a mass of 58 kg is standing on a square piece of wood that is 5 m wide and 7 m long. What is the pressure under the wood?
P = F / A F = m * g A = L * W

8. 7-1 Fluids and Pressure
A force of 479 N is on an area of 51 m2. What is the pressure?
P = F / A
P = 479 N / 51 m2
P = 9.39 N/m2
P = 9.39 Pa

9. 7-1 Fluids and Pressure
A force of 12 N is placed on a table that is 1 m wide and 3 m long. What is the pressure on the ground?
P = F / A A = L * W
A = 1 m * 3 m
A = 3 m2
P = 12 N / 3 m2
P = 4 N/m2
P = 4 Pa

10. 7-1 Fluids and Pressure
An object that has a mass of 457 g covers an area that is 3 cm by 6 cm. What is the pressure the object exerts on the surface?
An object exerts 45 Pa of pressure over an area of 35 m2. What is the mass of the object?
An object exerts 21 Pa of pressure and has a mass of 56 kg. What is the area that the pressure is on?

11. 7-1 Fluids and Pressure Fluid Pressure Fluid
Substance that can easily flow
Able to change shape
Liquids and gasses
Fluids exert pressure on the surfaces they touch

12. 7-1 Fluids and Pressure Fluid Pressure All Around Air Pressure
1 cubic meter of air weighs about 1 kg
Atmosphere is over 100 km high
10.13 N/cm2
Each square cm has N of force on it

13. 7-1 Fluids and Pressure Balanced Pressures
Remember air pushing down is also pushing up
Air pushing in is also pushing out
Pressures balance

14. 7-1 Fluids and Pressure Pressure and Depth
Pressure increases as depth increases

15. 7-1 Fluids and Pressure Pascal’s Principle
When force is applied to a confined fluid, an increase in pressure is transmitted equally to all parts of the fluid

16. 7-1 Fluids and Pressure Force Pumps
Cause fluid to move from one place to another by increasing the pressure in the fluid
Examples:
Hearts
Medicine droppers
Squeeze bottles

17. 7-1 Fluids and Pressure Using Pascal’s Principle
Small tube filled with water
Small piston can be used to move a car

18. 7-1 Fluids and Pressure Hydraulic Systems
Multiplies a force by applying the force to a small surface area
The increase in pressure is then transmitted to another part of a confined fluid, which pushes on a larger surface area

19. 7-2 Floating and Sinking Buoyancy Buoyant force
Force that acts in upward direction, against the force of gravity so it makes an object feel lighter
Pressure increases with depth so the upward pressure on bottom of object become greater then the downward pressure

20. 7-2 Floating and Sinking Archimedes’ Principle
States that the buoyant force of an object is equal to the weight of the fluid displaced by the object

21. 7-2 Floating and Sinking Density Density = mass / volume
An object that is more dense than the fluid in which it is immersed sinks
An object that is less dense than the fluid in which it is immersed floats to the surface

22. 7-2 Floating and Sinking Densities of substances Densities can change
Submarines change density to go up and down
Pop can change by releasing gas

23. 7-2 Floating and Sinking Buoyancy and Density Change the volume
Ship hulls are made mostly of air
Cause it to float

24. 7-3 Applying Bernoulli’s Principle
The faster a fluid moves, the less pressure the fluid exerts
States that the pressure exerted by a moving stream of fluid is less than the pressure of the surrounding fluid

25. 7-3 Applying Bernoulli’s Principle
Objects in Flight
Wings change shape to change air pressure
Slower air has more pressure than fast moving air so airplane rises
Spoiler pushes car to the ground

26. 7-3 Applying Bernoulli’s Principle
Bernoulli’s Principle at Home
Chimneys
Smoke rises because of wind blowing across top of chimney
Wind lowers the air pressure
High pressure in fire place pushes smoke upward