1. Fluids and Pressure
PHYS 1090 Unit 5

2. Balloon Mash
Greater force makes larger contact area

3. Pressure Force applied per unit area p = F/A
If pressure is constant, force and area change together

4. Pressure within Fluids
Pascal’s Principle: fluids exert pressure evenly in all directions

5. Balloon Mash
Upward force (pressure  area) on plate exactly cancelled plate’s weight
As weight increased, contact area did too
(pressure may have increased as well)

6. Fountain Water shoots farther from lower holes
Streams weaken as water drains

7. Static Fluids
Pressure counteracts weight of fluid above (Pascal’s principle)
Pressure increases with depth
Supports weight above

8. Liquid Pressure Formula
p = rhg h
p = pressure
r = density of liquid
h = depth under top of liquid
p = pressure here

9. Pressure within a Liquid
p = rhg
Shape of the container does not matter!
All that matters are depth h, fluid density r, and gravitational field g.

10. Fountain Stream velocity depends on pressure Pressure depends on depth
Rocks in the can have no effect

11. Sinking and Floating
Objects displace a volume of water equal to their submerged volume
A floating boat displaces an additional volume of air

12. What forces are present?

13. What forces are present?

14. What forces are present?

15. What forces are present?

16. Pressure in a fluid Pressure increases with depth
Greater pressure at bottom than top of an immersed object
Results in upward buoyancy force that is the (vector) sum of all pA forces

17. Buoyancy Force
Buoyancy force = weight of fluid displaced (Principle of Archimedes)
F = rVg
r = density of fluid
V = volume of fluid displaced = volume of object submerged
g = 9.8 N/kg

18. Sinking and Floating All objects are lighter under water
Difference is buoyancy force
If buoyancy > weight, object rises to surface and floats (so buoyancy = weight)
if buoyancy < weight, object sinks

19. Clay Lump Weight of the lump was constant
Making a boat increased the volume of water displaced
That increased the buoyancy force
A great enough buoyancy floated the boat

20. Expanding and Contracting
The same amount of gas occupies more volume at a higher temperature.

21. Convection Warm fluids expand, becoming less dense
Circulation is driven by buoyancy forces
Much faster than conduction

22. “Ideal Gas” Law pV = NkBT p = pressure V = volume
N = number of gas molecules
kB = 10–23 J/K
T = absolute (Kelvin) temperature

23. Gas Pressure and Volume
pV = NkBT
At a constant temperature, pV is constant
Increasing p decreases V and vice versa p V

24. Balloon Mash
Pressure may have increased with greater force because air was compressed (volume became less)

25. Diver Diver’s weight = weight of (glass + air)
Buoyancy = weight of excluded water
Increasing pressure decreases air volume
Buoyancy decreases
Weight is unchanged

26. Neutral Buoyancy Air doesn’t weigh much Glass weight doesn’t change
At neutral buoyancy SF = 0, buoyancy↑ = glass weight↓
Neutral buoyancy air volume is the same for all initial bubble sizes