Newton' First and Third Law

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Newton' First and Third Law
By Asha Saujani Krupa Patel Erin Parcell
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Newton's First Law
The Law of Inertia

• An object at rest tends to stay at rest and an
  object in motion tends to stay in motion with
  the same speed and in the same direction unless
  acted upon by an unbalanced force.

resisting state of motion

• Simple Terms
• things keep doing what they were doing until they
  were interrupted

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one more time.....

• Once moving at a steady speed
• In a straight line
• It will continue moving.
• At a steady speed.
• In a straight line..

Once standing still it will stay still..
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Rules

• Objects resist accelerations.
• Left to themselves, objects don't speed up, don't
  slow down, and don't change direction.
• It requires an unbalanced force to change the
  velocity of an object.
• If an object has a constant velocity, then no
  unbalanced forces are pushing on it.
• A force on an object tends to change it velocity.
  If no force pushes or pulls on an object (or if
  the forces on the object cancel each other out)
  then the object's velocity stays the same.
• No unbalanced force means no acceleration.
• No force means no acceleration.

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• If no forces act on an object (or if all of the
  forces that do act cancel each other out), then
  the object will not
• speed u,slow down, change directions
• If an object is not accelerating, then either
• no forces are pushing on it, or
• all of the forces that are pushing on it balance
  each other exactly.  
• Objects made of matter have inertia.
• The velocity of a free body is constant in time.
  (David Layzer, Constructing the Universe,
  Scientific American Library)
• If Fnet 0, then a 0.
• If there is no net force on an object it is in
  equilibrium, and vice versa.

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Inertia

• Newton's First Law says that any object made of
  matter has the property that it resists
  accelerations. The name of this property is
  inertia - inertia is the property of matter that
  causes objects to not "want to" accelerate. It is
  important to understand that inertia is not a
  force - inertia does not push an object along.
  Inertia is a property of matter, like color,
  density, etc.
• all objects made of matter obey Newton's First
  Law - regardless of how much inertia they have.
• Newton's First Law does not depend on how much
  inertia an object has - all objects made of
  matter have inertia, and they all obey Newton's
  First Law. In a little while, when we get into
  Newton's Second Law, the amount of inertia an
  object has will become important - but not now.

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

• can change velocity of an object..
• Examples
• two forces act on a object at one time the
  force to the left is strong so the net force on
  the left will cause the object to change velocity

it may appear as is there only one force at
acting on the object they are two separate
forces but one is stronger
What are the two forces?
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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 pounds, Newtons

What is an "object"
Anything made of matter is an object.
Equilibrium
An object that is not accelerating is said to be
in equilibrium.If an object is in equilibrium,
the net force on it is zero.
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Riding in a Car

• Accelerating from Rest - "G Forces"
• Situation
• Your car is at rest. Suddenly you push down on
  the "accelerator" and the accelerates forward.
  You feel that you are pushed back into your seat.
• Common Explanation
• "G Forces", caused by the acceleration, push you
  back into your seat.

• Problems with the Common Explanation
• First of all, there is nothing pushing you back
  in your seat! (Acceleration is a concept, not an
  object, and only objects can push you. See
  Thoughts on Force... for more discussion.)
• Correct Explanation
• You were at rest,
• you remain at rest.
• The car accelerates
• forward, and you
• stay where you are.

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Decelerating

• Common Explanation
• You are thrown forward.
• Problems with the Common Explanation
• What throws you?
• Correct Explanation
• You are moving with a constant velocity. You keep
  that velocity (while the car slows down) until
  something stops you.

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POP QUIZ!!!
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Newtons Third Law

• For every action,
• there is an equal and
• opposite reaction.

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Applying This Law to Life

• While you are sitting
• in your chair, your body
• exerts a downward force
• on the chair and the chair
• exerts an upward force
• on your body.

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Forces

• Forces always come in pairs.
• The direction is opposite.
• They are equal in size.
• A baseball forces the bat to the left
• and the bat forces the ball to the right,
• but they exert the same amount of force.

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Action and Reaction Forces

• Action and Reaction are names of forces.
• For every action force, there is a reaction
  force meansForces ALWAYS occur in
  pairs.Single forces NEVER happen.
• Since a force is an interaction between objects,
  two objects are involved in every force. Call the
  objects A and BAction force A pushes
  BReaction force B pushes A

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Equal

• In Newtons Third Law, equal meansEqual in
  size.The action and reaction forces are EXACTLY
  the same size.Equal in time.The action and
  reaction forces occur at EXACTLY the same time.

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Mass

• The same force acting on objects of different
  mass will produce different accelerations!
• FnetFnetmam aSameForce

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Internal Forces

• Internal ForcesIt is possible for Newtons Third
  Law action/reaction forces to cancel - if they
  act on different parts of the same object.These
  forces are called internal forces.

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Pop Quiz
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THE END