Chapter 12: Gravity, Friction, and Pressure

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

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?

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