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
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.
The Law of Inertia
resisting state of motion
•Simple Terms:
things keep doing what they
were doing until they were
interrupted

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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7.
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.
8.
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:
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9.
If an object is not accelerating, then either:
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10.
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speed u,slow down, change directions
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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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.


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
The net force on an object
is the vector sum of all of
the forces that push or pull
on the object.
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?

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
Anything made of matter is an “object”.
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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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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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.
http://www.batesville.k12.in.us/physics/PhyNe
t/Mechanics/Newton1/First_Law_Quiz.html#F
irst%20Law%20Quiz
Newton’s Third Law
 For
every action,
there is an equal and
opposite reaction.
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.

Forces
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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” are names of forces.
“For every action force, there is … a reaction
force” means:
Forces 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 B:
Action force: “A pushes B”
Reaction force: “B pushes A”

In Newton’s Third Law, “equal” means:
Equal 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.

The same force acting on objects of
different mass will produce different
accelerations!

FnetFnetma=m= aSame
Force

Internal ForcesIt is possible for
Newton’s Third Law action/reaction
forces to cancel - if they act on
different parts of the same object.
These forces are called “internal
forces”.
http://www.batesville.k12.in.us/physics/P
hyNet/Mechanics/Newton3/Third_Law_
Quiz.html