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
density =
mass
volume

3. Pressure
• A measure of the amount of force exerted
on a surface area
pressure =
force
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
P1V1 = P2V2

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