Newton's First Law

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Newton's First Law
Concept Summary Batesville High School Physics
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Aristotles View

• Two types of motion
• Natural motion - what an object naturally wants
  to do
• Violent motion - what an object has to be forced
  to do

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Aristotles View

• In order for a normal object to move at
  constant velocity, something must be pushing on
  it. When the pushing stops, the object (perhaps
  gradually) comes to a stop.

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Galileos View

• Realized that the classical view of motion did
  not recognize the role of friction
• If left to themselves, moving objects dont slow
  down.

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Newtons First Law

• Whatever an object is doing, thats what it wants
  to keep doing.
• If left to themselves, objects will keep doing
  whatever theyre doing.

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Newtons First Law

• Objects at rest tend to stay at rest.
• Objects in motion tend to stay in motion.

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Newtons First Law

• If there is no net force on an object, the object
  wont accelerate.

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Newtons First Law

• The converse is also true
• If an object is not accelerating, then there is
  no net force on it.

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What is an object?

• Anything made of matter is an object.

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What is a force?

• A force is an interaction between 2 objects
  involving a push or a pull.
• Forces are vectors - they have a direction in
  space.
• Common units of force are pounds, Newtons, or
  dynes.

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What is a net force?

• The net force on an object is the vector sum of
  all of the forces that push or pull on the object.

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no net force on an object means

• either there are no forces on the object, or
• the forces that push or pull on the object all
  cancel exactly.

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object wont accelerate means

• the object
• wont speed up.
• wont slow down.
• wont change direction.

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Newtons First Law can be stated

• If no forces push or pull on an object, or if the
  forces that do push or pull on it all cancel
  exactly, then the object will not speed up, slow
  down, or change direction.

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Inertia

• Newtons First Law says that objects do not
  accelerate spontaneously.
• This property of matter, which causes objects to
  resist acceleration, has been named inertia.
• Newtons First Law is often called the Law of
  Inertia.

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Equilibrium

• An object that is not accelerating is said to be
  in equilibrium.
• If an object is at rest and not accelerating, it
  is said to be in static equilibrium.

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Newtons First Law Again

• If an object is in equilibrium, the net force on
  it is zero, and
• If the net force on an object is zero, the object
  is in equilibrium.

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Friction

• Friction is a force that arises due to the
  relative motion of two surfaces.
• Two solid surfaces - sliding friction, rolling
  friction
• A solid and a fluid - air resistance
• Two fluids

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Friction

• The direction of the friction force always
  opposes the relative motion of the surfaces.

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Friction

• The amount of sliding friction depends on
• The condition (smoothness/roughness) of the
  surfaces
• The normal force between the surfaces

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The End

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Friction

• The amount of sliding friction does not depend
  on
• Area of contact
• Relative speed