1. Pressure; Pascal’s Principle

2. PRESSURE OF A FLUID pressure = height of mercury air pressure column
Barometer

3. Gauge pressure – pressure
above the atmospheric pressure.
absolute (total) pressure =
gauge pressure + atmospheric press.

5. Pressure is produced by the
weight of the fluid above the
surface.

6. force
pressure =
area
weight of fluid =
area mg
= -----
area
density · volume · g =
area
density · (area · height) · g =
area

7. pressure = density · height · g
P = ρhg gauge pressure
P = ρhg + P0 absolute pressure
SI units:
pressure: N/m2 = Pascals (Pa)
density: kg/m3
height (depth): m

8. Standard atmospheric pressure
760 mm Hg ≈ 30 in Hg
Calculate 1 atm in Pascals
P = ρgh
ρHg = 13.6 x 103 kg/m3
g = 9.8 N/kg h = m
P = x 105 Pa

9. Why does a barometer use mercury and not water?
If p = 1 atmosphere = x 105 Pa
ρ = 1.00 g/cm3 = 1.00 x 103 kg/m3
Find height.
10.3 m

10. Pressure of fluid depends on depth.

11. Pressure does not depend on volume,
only on height (depth).
“Water seeks its own level”

12. More pressure at greater depth

13. Measure blood pressure at upper arm
same height as heart

14. Pascal’s Principle – The pressure in an enclosed fluid is constant
throughout the fluid.
p = p2
F F2 =
A A2

15. p1 = p2 1 N F2 50 N ------- = ------ 1 cm2 50 cm2 1 N ? A = 1 cm2=
1 cm cm2
50 N

16. If piston on left moves 10 cm,
what distance does piston on right
move?
work1 = work2
F1 d1 = F2 d2
(1) (10) = (50) d2
0.2 cm

17. Hydraulic lift

18. Area of brake cylinder > area of brake line
force of brake cylinder > force of brake pedal

19. hydrostatic pressure