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Concept Summary
Batesville High School Physics
Aristotle’s 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
Aristotle’s 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.
Galileo’s View
 Realized
that the classical view of
motion did not recognize the role of
friction
 If left to themselves, moving objects
don’t slow down.
Newton’s First Law
 Whatever
an object is doing, that’s what
it wants to keep doing.
 If left to themselves, objects will keep
doing whatever they’re doing.
Newton’s First Law
 Objects
at rest tend to stay at rest.
 Objects in motion tend to stay in motion.
Newton’s First Law
If
there is no net force on an
object, the object won’t
accelerate.
Newton’s First Law
The converse is also true:
 If an object is not accelerating, then
there is no net force on it.
What is an “object”?
 Anything
“object”.
made of matter is an
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.
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.
“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.
“object won’t accelerate”
means:
 the
object:
won’t speed up.
won’t slow down.
won’t change direction.
Newton’s 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.
Inertia
 Newton’s
First Law says that objects do
not accelerate spontaneously.
 This property of matter, which causes
objects to resist acceleration, has been
named “inertia”.
 Newton’s First Law is often called the
Law of Inertia.
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.”
Newton’s 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.
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
Friction
 The
direction of the friction force always
opposes the relative motion of the
surfaces.
Friction
 The
amount of sliding friction depends
on:
 The
condition (smoothness/roughness) of
the surfaces
 The normal force between the surfaces
The End
Friction
 The
amount of sliding friction does not
depend on:
 Area
of contact
 Relative speed