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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Sticky Sneakers LabFriction depends on the
kinds of surfaces involved and how hard the
surfaces push together
Sneaker Starting Friction (N) Sideways-Stopping Friction (N) Forward-Stopping Friction (N)
A
B
C
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Challenge!
A jet engine generates 160 kN of force as it
propels a 20,000kg plane down a runway. If 40 kN
of friction opposes the plane, how much time it
will take the plane to reach a speed of 33m/s
from rest?
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Forces and Surfaces

• A flat surface may look and feel smooth, but it
  has tiny bumps and ridges which lead to friction
• Types of surfaces
• Depends on the materials that make up the
  surfaces puck on ice vs floor
• Motion of the Surfaces
• Apply a force to start an object moving friction
  increases to keep it from sliding
• Frictional force has a limit to how large it can
  be, and with enough force you can make the object
  move

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12.2 Friction is a force that opposes motion

• Friction occurs when surfaces slide against each
  other
• Easier to push a box over tile than carpet
• Friction force that resists the motion between
  two surfaces in contact
• Friction between your feet and ground
• Easier to exert a backward force on rougher
  surfaces, with the reaction force moving you
  forward more than a slick surface
• Provides the action and reaction forces that
  enable you to walk

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Forces and Surfaces

• Force pressing the surfaces together
• The harder two surfaces are pushed together, the
  more difficult it is for the surfaces to slide
  over each other
• An object has weight, and the surface exerts an
  equal and opposite reaction force on the object
  this increases friction force
• Friction depends on the force pressing the
  surfaces together, NOT the surface area over
  which the forces act

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Friction and Heat

• Friction between surfaces produces heat
• Rub your hands together
• Your energy is transferred to the individual
  molecules on the surface of your hands, causing
  them to move faster, therefore increasing the
  temperature
• Strike a match
• Brakes

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Motion through fluids produces friction

• Recall two objects falling in a vacuum fall with
  the same acceleration
• In air this is different air is a fluid, a
  substance that can flow easily
• An object moving through a fluid pushes the
  molecules of the fluid out of the way
• The molecules of the fluid exert an equal and
  opposite force on the object, slowing it down
• drag
• Depends on the shape of the moving object
• Air resistance depends on the surface area and
  speed of the object (different than friction)
• Larger surface area comes into contact with more
  molecules than a smaller surface increase
  surface are, increase air resistance
• Faster moving object comes into contact with more
  molecules in a given amount of time increase
  speed, increase air resistance

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Terminal Velocity

• Skydiver eventually the air resistance balances
  gravity terminal velocity

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Friction Simulation

• Projects
• _MS-Science
• PS-8th
• forces-and-motion_en.jar
• ramp-forces-and-motion_en.jar
• http//phet.colorado.edu/en/simulation/forces-and-
  motion
• http//phet.colorado.edu/en/simulation/ramp-forces
  -and-motion