1. More on Fluids and pressure

2. Fluids vs Solids Solid: Has a definite shape and volume Fluids:
Includes liquids and gases
Take on the shape of their container (no definite volume)
Liquids: no definite shape but have a definite volume
Gases: no definite shape or volume

3. Fluids vs Solids – Particle Model
-Neatly arranged
-Strong forces of attraction between particles (physical bonds)
-No real movement of particles
-Close together but not as structured as a solid
-Weak forces of attraction between particles
-Particles can slide over each other
-Far apart
-No forces of attraction between particles
-Move a lot!

4. Compressible vs Incompressible Fluids
Their volume can be changed as a response to pressure
They can be squished
Incompressible fluids
Their volume does not change as a response to pressure
They cannot be squished

5. Compressible vs Incompressible
Add Pressure
Compressible!
Add Pressure
Incompressible!

6. Pressure
Pressure is the result of a force applied in a perpendicular fashion to a surface
Pressure is therefore affected by:
The amount of force applied
Increased force =
The surface area
Increased surface area =
Increased pressure
Decreased pressure

7. 𝑃= 𝐹 𝐴 Pressure P is pressure, usually measured in Pascals (Pa)
𝑃= 𝐹 𝐴
P is pressure, usually measured in Pascals (Pa)
F is force measured in Newtons (N)
A is the surface area measured in m2

8. Pressure In m2 𝑃= 𝐹 𝐴 A= 𝑐 2 10N A= (0.4𝑚) 2 A= 0.16𝑚 2
1) What is the pressure exerted by the following box?
In m2
𝑃= 𝐹 𝐴
A= 𝑐 2
10N
A= (0.4𝑚) 2
A= 0.16𝑚 2
𝑃= 10𝑁 0.16 𝑚 2
40cm
𝑃=62.5𝑃𝑎
40cm

9. Pressure In m2 𝑃= 𝐹 𝐴 A= 𝑐 2 A= (0.2𝑚) 2 10N A= 0.04𝑚 2
2) What is the pressure exerted by the following box?
In m2
𝑃= 𝐹 𝐴
A= 𝑐 2
A= (0.2𝑚) 2
10N
A= 0.04𝑚 2
𝑃= 10𝑁 0.04 𝑚 2
20cm
𝑃=250𝑃𝑎
20cm

10. Pressure Area ↓ Pressure ↑ 10N 10N 𝑃=62.5𝑃𝑎 𝑃=250𝑃𝑎
If we keep the force the same but change the surface area….
10N
10N
Area ↓
Pressure ↑
20cm
40cm
20cm
40cm
𝑃=62.5𝑃𝑎
𝑃=250𝑃𝑎

11. Pressure and Fluids
In incompressible fluids (aka liquids) pressure is a result of the force exerted by the mass of the particles
The more liquid above an object, the more mass above the object, therefore the more force exerted and the greater the pressure
Pressure is also dependent on density
↑ density = ↑ pressure

12. Pressure and Fluids
In compressible fluids (aka gases) pressure is a result of the force exerted by the particles when they collide with an object or each other. The sum of all of these forces = pressure
Pressure is dependent on the number of collisions. More collisions = higher pressure
Collisions are dependent on:
Number of particles
Temperature
Volume

13. Pressure and Fluids Collisions are dependent on: Number of particles
↑ particles = ↑ pressure
Temperature
↑ temperature = ↑ pressure
Volume
↑ volume = ↓ pressure

14. If the air is exerting pressure on us all the time, why aren’t we being crushed?

15. Air pressure
If the air is exerting pressure on us all the time, why aren’t we being crushed?
The air particles are hitting us from all sides equally and so their effects cancel each other out!

16. Principles of Fluid Pressure
First Principle:
Fluids move from areas of HIGH pressure to LOW pressure
Think about when we breathe!
This principle is also applied in the circulatory system

17. Principles of Fluid Pressure
Second Principle:
Pressure applied to the surface of a fluid inside a closed container gets uniformly distributed to every part of the fluid
Ex: pressing on a tube of toothpaste!
Or a syringe

18. Principles of Fluid Pressure
Third Principle:
The transfer of pressure in a fluid can increase the force involved
The carjack!

19. Principles of Fluid Pressure
As you press on the small piston you transfer pressure (principle 2)
Let’s say 20 Pa
This same pressure gets transferred to the other side but is now spread over a larger area

20. Principles of Fluid Pressure
Small piston:
𝑃= 𝐹 𝐴
𝑃= 20𝑁 0.1 𝑚 2
P = 200 Pa
Large piston:
F=𝑃𝐴
F= (200 Pa)(1 𝑚 2 )
F = 200 N
Force of 20N became 200N!

21. Homework
Textbook: p #4, 6, 7, 8, 12, 14, 16 & 21