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Types of Friction

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What is Friction?

  • Friction is the resistance to the sliding, rolling, or flowing motion of an object due to its contact with another object. 

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  • When a moving object comes in contact with another object, friction is the opposing force that slows or stops the moving object.

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  • Without friction, an object would continue to move at constant speed forever.

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  • The strength of the force of friction depends on the type of surfaces and how hard the surfaces push together.

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  • Rough surfaces produce greater friction than smooth surfaces.

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  • Without friction you could not
    • Write
    • Drive a car
    • Fly an airplane

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  • Why not?

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Four Types of Friction

  • Rolling Friction
  • Static
  • Sliding Friction
  • Fluid Friction

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What do all these things have in common?

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Rolling Friction

  • Rolling friction is friction that occurs between surfaces in motion in which one of the surfaces is a wheel, roller, or ball.
  • Examples:
    • Riding a bike – tires and ground
    • Bowling – ball and lane

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What do all these things have in common?

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Static Friction

  • Static friction is friction that occurs when the surfaces in contact are at rest (not in motion).

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  • Examples:
    • A book resting on a desk.
    • A potted plant sitting on a sidewalk.

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What do all these objects have in common?

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Sliding Friction

  • Sliding friction is friction that occurs when solid surfaces slide over one another.
  • Examples:
    • Writing – pencil point and paper
    • Combing your hair – surface of comb and strands of hair

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What do all these things have in common?

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Fluid Friction

  • The force that tries to slow objects down when they move through a liquid or a gas. It's also known as "drag", or "air resistance".
  • All gases and liquids are fluids.
  • An airplane and a swimmer both experience fluid friction.

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Ways to Reduce Friction

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  • Smooth the surface
  • Replace rolling with sliding
  • Add oil or another type of lubricant

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Friction Song

  • What is friction?
    • Friction Video

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Gravity and Free Fall

  • An object is in free fall when the only

force acting on the object is gravity.

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  • Free-falling objects do not encounter

air resistance.

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  • In free fall, the force of gravity is an

unbalanced force.

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Calculating Free Fall

All objects in free fall accelerate at the same rate – 9.8 m/s² - regardless of their mass.

in one second = 9.8 m/s²

in two seconds = 19.6 m/s²

in three seconds = 29.4 m/s²

in four seconds = ________

The velocity continues to increase as the object falls.

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Gravity, Free Fall, and Projectile Motion

  • An object that is thrown is called a projectile.

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  • If there is no air resistance, an object that is dropped and an object that is thrown are both in free fall.

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  • Without the force of air resistance, both objects will hit the ground at the same time.

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Gravity and Air Resistance

  • Most objects falling through air experience a type of fluid friction called air resistance.
  • Air resistance is not the same for all objects.
  • The greater the surface area of the object the greater the air resistance.
  • However, since the elephant has more mass, it has more downward force of gravity and falls faster.

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What type of friction?

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  • Static Friction

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What type of friction?

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  • Rolling Friction

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What type of friction?

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  • Fluid Friction

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What type of friction?

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  • Static Friction

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What type of friction?

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  • Sliding Friction

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What type of friction?

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  • Fluid Friction