Friction and Gravity

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• Friction and Gravity

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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. 
• 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.
• The strength of the force of friction depends on
  the type of surfaces and how hard the surfaces
  push together.
• Rough surfaces produce greater friction than
  smooth surfaces.

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Three Types of Friction

• Sliding Friction
• Rolling Friction
• Fluid Friction

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Sliding Friction

• A resistance to movement that is created when two
  things rub together.

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Rolling Friction

• When an object rolls over a surface.
• Skateboard Example
• The wheels and ball bearings in the wheels are
  used to reduce friction between the floor and the
  skateboard.
• The skateboard will slow and stop due to the
  friction between the floor and the skateboard.

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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

• Smooth the surface
• Put ball bearings in wheels
• Replace rolling with sliding
• Add oil or another type of lubricant

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Gravity

• The force that pulls objects toward each other.

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Gravity and Free Fall

• An object is in free fall when the only
• force acting on the object is gravity.
• Free-falling objects do not encounter
• air resistance.
• 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.
• If there is no air resistance, an object that is
  dropped and an object that is thrown are both in
  free fall.
• 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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Universal Gravitation

• The law of universal gravitation states that the
  force of gravity acts between all objects in the
  universe.