Chapter 12: Gravity, Friction, and Pressure

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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

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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

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A Bed of Nails
                                                                                Would the pressurebe greater, less, or the same if there were only one nailin each board?
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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

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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
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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

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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

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Atmospheric Pressure
                          The weight of a one-square metercolumn of air 30 miles tall isabout 100,000 N, or about25,000 pounds.----------------------------------------------Why doesn't the pressure of theatmosphere break windows?----------------------------------------------At what temperature would water boil in a vacuum?----------------------------------------------
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