Chapter 12: Gravity, Friction, and Pressure - PowerPoint PPT Presentation

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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 – PowerPoint PPT presentation

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Title: 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
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
3
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?
4
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

5
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

6
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

7
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

8
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

9
Terminal Velocity
  • Skydiver eventually the air resistance balances
    gravity terminal velocity

10
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
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