Physics 2

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 contd.

• Other units for pressure
• P lbs / in2 psi
• P torr 133.3 Pa or 1/760 atm
• P atm 1.01 x 105 Pa
• 14.7 lb/in2 which is
  pressure at
• sea level

4
Question

• At sea level the atmosphere presses down on us
  with a pressure equivalent to 14.7 pounds per
  square inch. Why arent we crushed by the air
  pushing in on us at every surface?

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?

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?

7
Pressure contd.

• 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.

9
Pressure contd.

• Pressure increases with depth
• Pressure at top surface equals F1/A
• Pressure at bottom surface equals F2/A
• SF P2A P1A mg
• Since fluid is static can show
• P2 P1 ?gh
• Only good if ? is same at any depth

10
Pascals 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 Pascals principle can show
• Poutside ?gh
• 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