1. Fluid Mechanics Liquids and gases have the ability to flow
They are called fluids
There are a variety of “LAWS” that fluids obey
Need some definitions

2. Density
Regardless of form (solid, liquid, gas) we can define how much mass is squeezed into a particular space

3. Pressure
A measure of the amount of force exerted on a surface area

4. Pressure in a Fluid
The pressure is just the weight of all the fluid above you
Atmospheric pressure is just the weight of all the air above on area on the surface of the earth
In a swimming pool the pressure on your body surface is just the weight of the water above you (plus the air pressure above the water)

5. Pressure in a Fluid
So, the only thing that counts in fluid pressure is the gravitational force acting on the mass ABOVE you
The deeper you go, the more weight above you and the more pressure
Go to a mountaintop and the air pressure is lower

6. Pressure in a Fluid
Pressure acts perpendicular to the surface and increases at greater depth.

7. Pressure in a Fluid

8. Buoyancy Net upward force is called the buoyant force!!!
Easier to lift a rock in water!!

9. Displacement of Water
The amount of water displaced is equal to the volume of the rock.

10. Archimedes’ Principle
An immersed body is buoyed up by a force equal to the weight of the fluid it displaces.
If the buoyant force on an object is greater than the force of gravity acting on the object, the object will float
The apparent weight of an object in a liquid is gravitational force (weight) minus the buoyant force

11. Flotation
A floating object displaces a weight of fluid equal to its own weight.

12. Flotation

13. Gases
The primary difference between a liquid and a gas is the distance between the molecules
In a gas, the molecules are so widely separated, that there is little interaction between the individual moledules
IDEAL GAS
Independent of what the molecules are

14. Boyle’s Law

15. Boyle’s Law Pressure depends on density of the gas
Pressure is just the force per unit area exerted by the molecules as they collide with the walls of the container
Double the density, double the number of collisions with the wall and this doubles the pressure

16. Boyle’s Law Density is mass divided by volume.
Halve the volume and you double the density and thus the pressure.

17. Boyle’s Law
At a given temperature for a given quantity of gas, the product of the pressure and the volume is a constant

18. Atmospheric Pressure Just the weight of the air above you
Unlike water, the density of the air decreases with altitude since air is compressible and liquids are only very slightly compressible
Air pressure at sea level is about 105 newtons/meter2

19. Barometers

20. Buoyancy in a Gas
An object surrounded by air is buoyed up by a force equal to the weight of the air displace.
Exactly the same concept as buoyancy in water. Just substitute air for water in the statement
If the buoyant force is greater than the weight of the object, it will rise in the air

21. Buoyancy in a Gas
Since air gets less dense with altitude, the buoyant force decreases with altitude. So helium balloons don’t rise forever!!!

22. Bernoulli’s Principle

23. Bernoulli’s Principle
Flow is faster when the pipe is narrower
Put your thumb over the end of a garden hose
Energy conservation requires that the pressure be lower in a gas that is moving faster
Has to do with the work necessary to compress a gas (PV is energy, more later)

24. Bernoulli’s Principle
When the speed of a fluid increases, internal pressure in the fluid decreases.

25. Bernoulli’s Principle

26. Bernoulli’s Principle
Why the streamlines are compressed is quite complicated and relates to the air boundary layer, friction and turbulence.

27. Bernoulli’s Principle