1. Physics 2
Chapter 9
Section 2

2. Pressure
Ratio of the magnitude of the force acting perpendicular to the surface to the area over which the force acts
P = F / A [P] = N/m2 = Pascal, Pa
since Pa is very small pressure is usually measured in kPa

3. Pressure cont’d. Other units for pressure [P] = lbs / in2 = psi
[P] = torr = Pa or 1/760 atm
[P] = atm = 1.01 x 105 Pa =
14.7 lb/in2 which is pressure at
sea level
All fluids exert pressure perpendicular to the surface they are in contact with but never at an angle to the surface
If a fluid exerted a force tangent to a surface the surface would push back and cause the fluid to flow (can’t if static)

4. Question
At sea level the atmosphere presses down on us with a pressure equivalent to 14.7 pounds per square inch. Why aren’t we crushed by the air pushing in on us at every surface?
Bodies are relatively permeable to air so our internal pressure stays the same as the external pressure (inward push = outward push)
Usually notice pressure when it changes
Ex – go up a mountain where air P drops your ear P is greater so yawn to open Eustachian tubes
Ex – diving the pressure increases so have to force air into ears to balance pressure

5. Sample Problem A man weighs 200 lb. At one particular
moment when he walks, his right heel is the
only part of his body that touches the
ground. If the heel of his shoe measures
3.5 in. by 3.25 in., what pressure does the
man exert on the ground?
17.6 lb/in2

6. Sample Problem A woman weighs 100 lb and is wearing high-
heeled shoes. The cross section of her
high-heel shoe measures ½ in. by 5/8 in.
At a particular moment when she is walking,
only one heel of her shoe makes contact
with the ground. What is the pressure
exerted on the ground by the woman?
320 lb/in2
Examples of pressure –
Snowshoes (large A so F/A is small)
Bed of nails
Cola can
Candle and beaker
egg and flask
Papers and sticks

7. Pressure cont’d.
Pressure in water is created by the weight of the water
Pressure in air is created by the air being pulled down by gravity toward the center of the earth
Air pressure (like water pressure) increases with depth

8. Sample Problem
An airplane window has an area of 825 cm2. Cabin pressure is 1 atm and the outside pressure is 0.3 atm. Find the net force produced by air pressure on the window. N

9. Pressure cont’d. Pressure increases with depth
Pressure at top surface equals F1/A
Pressure at bottom surface equals F2/A
ΣF = P2A – P1A – mg
Since fluid is static can show
P2 = P1 + ρgh
Only good if ρ is same at any depth

10. Pascal’s Principle
Any change in the pressure applied to a completely enclosed fluid is transmitted undiminished to all parts of the fluid and the enclosing walls
Since P1 = P2 the F2 = F1 (A2/A1)
Principle behind hydraulic lifts and hypodermic needles

11. Barometer
1643, Torricelli invented the barometer to measure atmospheric pressure
Filled a glass tube with Hg and inverted it in a container of Hg
When Hg drop in tube it creates a vacuum
Using Pascal’s principle can show
Poutside = ρgh
Pressure is indicated by the height of Hg

12. Gauge Pressure
Amount by which the container pressure exceeds atmospheric pressure
Ex – tire gauge
if car tire has a gauge pressure of 2 atm its actual pressure is 3 atm