Concept Summary
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Mario Mendoza
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Newton’s Third Law
For
every action, there is an
equal and opposite reaction.
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Mario Mendoza
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What are “action” and
“reaction”?
 “Action”
and “Reaction” are names
of forces.
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Mario Mendoza
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“For every action…reaction”
 “For
every action force, there is …
a reaction force” means:
Forces ALWAYS occur in pairs.
Single forces NEVER happen.
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Mario Mendoza
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Action & Reaction Forces
 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”
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“Equal”
 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.
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Mario Mendoza
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“opposite”
 In
Newton’s Third Law, “opposite”
means:
 Opposite in direction
 The action and reaction forces are
EXACTLY 180o apart in direction.
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Two Logical Difficulties
1.
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If Newton’s Third Law action &
reaction forces are equal and
opposite, how come they don’t always
cancel, making net force and
acceleration impossible?
(This is the question in the “Horse and
Wagon” problem.)
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The Keys to Understanding
 Only
forces pushing or pulling on an
object affect the object’s motion.
Only
forces that act on the
same object can cancel.
 Newton’s
Third Law action and reaction
forces push/pull on different objects,
so they don’t cancel.
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Mario Mendoza
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Action & Reaction Again
 The
(action) force “A pushes B” affects
the motion of object B.
 The (reaction) force “B pushes A”
affects the motion of object A.
 The
action and reaction forces don’t
cancel since they push on different
objects.
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Mario Mendoza
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The Other Logical Problem
2.
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If the Newton’s Third Law action and
reaction forces are always equal and
opposite, how do two objects of
different sizes get different
accelerations in the same interaction?
(When a bug hits a windshield,
different things happen to the bug and
windshield.)
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The Second Difficulty
Addressed
 If
the action and reaction forces are the
same size, how can two objects push
on each other and get different
accelerations?
 Newton’s Second Law says that the
acceleration of an object depends not
only on the force on it, but on the
object’s mass.
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Mario Mendoza
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Don’t Forget Mass!
 The
same force acting on objects of
different mass will produce different
accelerations!
Same
Force
Fnet
m
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=
a
Fnet
m
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=a
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Internal Forces
 It
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”.
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Mario Mendoza
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Systems
 The
object or objects that you are
dealing with in a given situation is called
a “system”.
 How you pick your system determines
whether a particular action/reaction
force pair will cancel or not.
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Mario Mendoza
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Systems
 There
is no “trick” to picking a “correct”
system - Newton’s Laws will work just
fine in any system.
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Mario Mendoza
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