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

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

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

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

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

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

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

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? ----------------------------------------------