3.  water at 4° C = 1.000 x 103 kg/m3 SI unit = kg/m3
 varies with temperature and pressure
Significantly for gases
Not much for liquids or solids
 water at 4° C = x 103 kg/m3
(We can use this value for water at any temperature.)
© Laura Fellman, PCC Rock Creek Campus

4. Pressure Box of air Total Force  to surface Notes on Pressure
SI unit = N/m2 = 1 Pascal (Pa)
1 Pa is a very small amount of pressure:
Normal air pressure  105 Pa = 1 bar
1 atmosphere (1 atm) = 1.01x105 Pa
= atmospheric pressure at sea level
1 mmHg = 133 Pa = 1 Torr
mm of mercury
© Laura Fellman, PCC Rock Creek Campus

5. One atmosphere (atm) is the average pressure exerted by the earth’s atmosphere at sea level
1.00 atm = 1.01 x105 N/m2 = kPa
What is the approximate mass of air above you in Mussoorie?

6. NZ Weather Maps

7. © Laura Fellman, PCC Rock Creek Campus
Fluids exert pressure on all surfaces they contact:
*Pressure is not a vector:
Total magnitude (without direction)
© Laura Fellman, PCC Rock Creek Campus

8. Why doesn’t the box collapse??
.20m
[1]Find the total force on the outside of the box due to 1 atm of air pressure.
Why doesn’t the box collapse??
[2] Estimate the pressure exerted on a floor by a 50 kg model standing momentarily on a single spiked heel (area = 0.05 cm2) and compare it to the pressure exerted by a 1500 kg elephant standing on one foot (area = 800 cm2).
Ans: 9.8 x 107 Pa; x 105 Pa
© Laura Fellman, PCC Rock Creek Campus

9. More pressure examples
Snowshoes:
Bed of nails:
© Laura Fellman, PCC Rock Creek Campus

10. Why does pressure increase with depth in a fluid, and by how much?
Pressure and Depth
Why does pressure increase with depth in a fluid, and by how much? P2 P1 h
Imagine a tank of water at rest.
We’ll focus on a column of this water.
Draw a free body diagram of the vertical forces on the column of water.
Since it is in equilibrium, the forces must sum to zero.
Solve for the pressure at the bottom of the column.
P2 = P1 + gh
Does pressure vary horizontally??
© Laura Fellman, PCC Rock Creek Campus

11. P2 P1 h

12. Static fluid Pressure does not depends on shape, mass or surface area of the liquid

13. Static fluid Pressure does depends on depth
Hydrostatic Pressure
Water Pressure Applet

14. Find the pressure at 100 m below sea level.
Ex:
Find the pressure at 100 m below sea level.
A newspaper article reports the pressure at the depth of the Titanic wreck as 6,600 pounds per square inch. Calculate its depth in km.

15. = 3.7x104 Pa PGAUGE = P PISTON Pabs = PPISTON + Patm = 1.38x105 Pa
A 30.0-kg piston holds compressed gas in a tank of volume 100 m3. The radius of the piston is m.
a. Determine the gauge pressure of the gas in the tank.
m = 30 kg
V = 100 m3
r = 0.05 m
= 3.7x104 Pa
PGAUGE = P PISTON
b. Determine the absolute pressure
Pabs = PPISTON + Patm
= 3.7x x105
= 1.38x105 Pa

16. © Laura Fellman, PCC Rock Creek Campus
Pressure Gauges
1) Mercury Barometer (measures atmospheric pressure)
density of mercury
P1 = 0 h
P2 = P1 + gh
Patm = mgh
P2 = Patm
© Laura Fellman, PCC Rock Creek Campus

18. Hydrostatic Pressure in Liquids (U-tube) 2) Open-Tube Manometer
Fluid in a manometer
Hydrostatic Pressure in Liquids (U-tube)
2) Open-Tube Manometer
(measures the pressure of
this substance)
P2 = P1 + gh
Psubst = Patm + mgh
P1 = Patm
Psubst - Patm = mgh h
= Gauge Pressure
= the amount that pressure differs from the atmosphere
P2 = Psubst.
mercury
© Laura Fellman, PCC Rock Creek Campus

19. Gauge Pressure – pressure relative to atmospheric pressure
Positive – above atmospheric pressure
Negative – below atmospheric pressure
Absolute Pressure – the true value of pressure applied to an object
= atmospheric pressure + gauge pressure

20. [3] A novice scuba diver has an extra 12 kilopascals of air pressure on her while underwater. What is her depth?
P = Po + ρ g h
Po = pascals so P = pascals
= (1000)(9.8) h
= 9800 h
ANSWER: h = 1.22 meter
[4] A tire gauge reads 220 kPa. What is the absolute pressure within the tire?
Ans: 321 kPa