1. Chapter 12: Gravity, Friction, and Pressure
12.1 Gravity is a force exerted by masses
12.2 Friction is a force that opposes motion
12.3 Pressure depends on force and area
12.4 Fluids can exert a force on objects

2. Pressure describes how a force is spread over an area
Pressure - a measure of how much force is acting on a certain area
Describes how “concentrated” a force is
Pressure = Force / Area P = F/A
Pressure equals force per unit area
Cat laying on lap, then stands up
Feels like more force, but force is still the same, just over a different area  different pressure

3. A Bed of Nails
                                                                               
Would the pressure be greater, less, or the same if there were only one nail in each board?

4. Pressure To increase pressure (P = F/A)
Increase force (F)
Decrease area (A)
Thumbtack: decreased area – can pierce the wall!
Snowshoes: can walk across snow and not sink!
Unit: Pascal (Pa) = pressure exerted by one newton (1 N) of force on an area of one square meter ( 1 m2)
1 Pa = 1 N/m2

5. Calculating
Hiker weighing 500 N is wearing snowshoes an area of 0.2 m2 s. What pressure is being exerted on the snow?
What do you know?
What do you want to find out?
Formula:
Substitute:
Calculate and simplify
Check units
Answer

6. Pressure in Air
Air does have weight (force), and so exerts a force on you at all times
Sea level pressure = 100,000 pascals = “1 atmosphere” = 1 atm
Every square centimeter of your body feels a force of 10 N (2.2 lb)
10N/cm2 = 10N/.0001 m2 =100,000N/m2 = 100,000 pascals

7. Pressure in Air Changing Elevation: air has mass, so it has weight
Higher elevation, shorter column of air above you, so less air, less mass, less force per same area = less pressure
Changing density: Air is compressible
Air at the top of a column presses down on the air below it, so is more dense (more molecules per volume (d = m/V)
Effects on Pressure: Denser air, increases collision and force  increase pressure
Increase elevation, pressure in your ear drum continues to push out with the same force that balanced the air pressure at lower elevations

8. Atmospheric Pressure
                         
The weight of a one-square meter column of air 30 miles tall is about 100,000 N, or about 25,000 pounds Why doesn't the pressure of the atmosphere break windows? At what temperature would water boil in a vacuum?