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Newton's 3rd Law Notes
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3rd Law in Action
Newton's 3rd Law Notes
Newton’s Third Law
For
every action, there is an
equal and opposite reaction.
What are “action” and
“reaction”?
“Action”
and “Reaction” are names
of forces.
“For every action…reaction”
“For
every action force, there is …
a reaction force” means:
Forces ALWAYS occur in pairs.
Single forces NEVER happen.
Newton's 3rd Law Notes
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”
force: “B pushes A”
Reaction
“Equal”
In
Newton’s Third Law, “equal” means:
in size.
Equal
The
action and reaction forces are
EXACTLY the same size.
Equal
in time.
The
action and reaction forces occur at
EXACTLY the same time.
“opposite”
In
Newton’s Third Law, “opposite”
means:
Opposite in direction
The action and reaction forces are
EXACTLY 180o apart in direction.
Newton's 3rd Law Notes
Scooter
Demonstration
Newton’s 3rd Law means…
Every action has a reaction
Every Reaction has a reaction
3. Every actions bring forth an opposite
action
4. Every action has an equal and
opposite reaction
1.
2.
Forces always occur in…
Triplets
Quadruplets
3. Singles
4. Pairs
1.
2.
Newton's 3rd Law Notes
Equal Means
Forces are the same size
Forces are the same size and time
3. Forces are the same time
4. Forces are the same shape
1.
2.
Opposite means
Forces are at 180 degree angles
Forces disagree with one another
3. The forces attract
4. The forces have opposing viewpoints
1.
2.
Two Logical Difficulties
1.
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.)
Newton's 3rd Law Notes
The Other Logical Problem
2.
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.)
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.
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.
Newton's 3rd Law Notes
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.
Don’t Forget Mass!
The
same force acting on objects of
different mass will produce different
accelerations!
Same
Force
Fnet
m
Scooter
=
a
Demo
Fnet
m
=a
Newton's 3rd Law Notes
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”.
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.
Systems
There
is no “trick” to picking a “correct”
system - Newton’s Laws will work just
fine in any system.
Newton's 3rd Law Notes
The End.