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Newton’s Laws
1:40-21:40
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6. Newton's Laws
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Newton’s Laws of Motion
First Law or Law of Inertia
A body at rest remains at rest and a body in motion continues to move at
constant velocity along a straight line unless acted upon by an external
force.
Second Law
A force action on a body gives it an acceleration which is in the direction of
the force and has a magnitude given by ma
Third Law
To every action there is always an equal and opposite reaction: or the
forces of two bodies on each other are always equal and are directed in
opposite directions.
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Inertial Frames
Inertia frame is any reference frame that is not accelerating
Any reference frame that moves with constant velocity relative to an inertial
frame is itself an inertial frame.
If you accelerate relative to an object in an inertial frame, you are observing the
object from a non-inertial reference frame.
A reference frame that moves with constant velocity relative to the distant
stars is the best approximation of an inertial frame.
Newton’s first two Laws can only
be applied in an Inertia Frame
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Newton’s First Law or Law of Inertia
It was Galileo who first expressed what he called
the law of inertia
"A body at rest remains at rest and a body in
motion continues to move at constant velocity
along a straight line unless acted upon by an
external force."
And Newton's own words in his famous book,
Principia.
"Every body perseveres in its state of rest or of
uniform motion in a right line unless it is compelled
to change that state by forces impressed upon it."
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Newton’s First Law
In the absence of external forces, when viewed from an inertial reference frame,
an object at rest remains at rest and an object in motion continues in motion with
a constant velocity.
 Newton’s First Law describes what happens in the absence of a net force
 Also tells us that when no force acts on an object, the acceleration
of the object is zero
 Can conclude that any isolated object is either at rest or moving at a constant
velocity
The First Law also allows the definition of force as that which causes a change
in the motion of an object.
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Inertia and Mass
The tendency of an object to resist any attempt to change its velocity is
called inertia.
Mass is that property of an object that specifies how much resistance an
object exhibits to changes in its velocity.
Masses can be defined in terms of the accelerations produced by a given
force acting on them:
m1
m2

a2
a1
 The magnitude of the acceleration acting on an object is inversely
proportional to its mass.
Which person would be easier to move?
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Mass vs. Weight
Mass and weight are two different quantities.
Weight is equal to the magnitude of the gravitational force exerted on the object.
 Weight is proportional to mass, but will vary with location and.
Example:
 wearth = 180 lb; wmoon ~ 30 lb
 mearth = 80 kg; mmoon = 80 kg
Mass
inherent property of an object.
independent of the object’s surroundings.
independent of the method used to measure it.
scalar quantity.
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Clicker Quiz
Consider a cart on a
horizontal frictionless
a) slowly come to a stop
table. Ignoring air
b) continue with constant acceleration
resistance, once the cart
c) continue with decreasing acceleration
has been given a push and d) continue with constant velocity
released, what will happen e) immediately come to a stop
to the cart ?
Newton’s Second Law
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