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Newton’s Third Law of Motion
– Action and Reaction
Chapter 7
Objectives
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Define force as part of an interaction.
State Newton’s third law of motion.
Describe how to identify a pair of action-reaction
forces.
Explain why the accelerations caused by an
action force and a reaction force do not have to
be equal.
Explain why an action force is not cancelled by a
reaction force.
Explain how a horse-cart system accelerates.
Explain what must occur in every interaction
between things.
7.1 Forces and Interactions
Recall (Ch. 2) that a force is a
push or pull.
 According to Newton, a force is
always part of a mutual action
that involves another force.
 This mutual action is called an
interaction.
 For example, a bat exerts a force
on a ball but the ball also exerts a
force on the bat.
 So for every interaction, there are
a pair of forces involved.
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7.2 Newton’s Third Law
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According to Newton,
whenever one object exerts a
force on a second object, the
second object exerts an equal
and opposite force on the
first object.
• One force is called the action
force.
• The other force is called the
reaction force.
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In other words, for every
action, there is an equal and
opposite reaction.
7.2 Newton’s Third Law
Important points to remember:
 In every interaction, the forces always
occur in pairs.
 The action/reaction forces are always
equal in magnitude.
 The action/reaction forces are always
opposite in direction.
Examples
The girl
pushes on
the wall and
the wall
pushes on
her.
The athlete pushes the bar upwards.
The bar pushes the athlete
downwards.
The swimmer pushes backward on the
water and the water pushes forward on her
7.2 Newton’s Third Law
Notice that interactions
generally depend on
friction.
 That is, if an action force
cannot be exerted
(because the surface is
ice, for example), there
cannot be a reaction
force.
 There is no resulting
forward motion.
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7.3 Identifying Action and Reaction
In the examples given previously, a pattern for
identifying the action/reaction pair is apparent.
 First, identify the objects “A” and “B”, then follow
this simple “recipe”:
ACTION: A exerts a force on B
REACTION: B exerts on force on A
 Example:
ACTION: foot pushes
against sand
REACTION: sand pushes
against foot
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Practice
A car is cruising
down the street.
Bowling ball pushes pin leftwards
A rocket is lifting
off toward space.
.
Baseball is caught in glove.
A ball is dropped off
a very tall building.
.
A sledgehammer drives a stake.
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Determine the action-reaction forces of the
above interactions.
Check Your Understanding
While driving, Anna observed a bug
striking the windshield of her car. The bug
hit the windshield and the windshield hit
the bug. Which of the two forces is
greater: the force on the bug or the force
on the windshield?
 We know that the Earth pulls on the
moon. Does the moon also pull on the
Earth? If so, which pull is stronger?
 In tug-of-war, if you pulling on the rope is
the action force, is the reaction force the
ground pushing back on you or your
opponent pulling back on the rope?
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7.4 Action and Reaction on
Different Masses
In the interaction between the
boulder and Earth, the boulder
will push on the Earth with as
much force as the Earth will push
on the boulder. The FORCES will
be the same.
 The masses of the 2 are quite
unequal, however.
 Since a = F/m, the boulder (low
m) will experience a much
greater acceleration than the
Earth (high m).
 The Earth DOES accelerate also
but we cannot “sense” it because
the acceleration is infinitesimally
small.
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Force and Mass
Think about the firing of
a cannon.
 The force the cannon
exerts on the cannonball
is exactly equal and
opposite to the force
exerted by the
cannonball on the
cannon.
 The cannonball moves
forward and the cannon
“kicks” or recoils. They
both accelerate but not
by the same amount due
to their differing masses.
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Force and Mass
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Let F represent both the action & reaction forces.
 M represents the mass of the cannon and m
represents the mass of the ball.
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For the cannonball: F =
For the cannon: F
m
=a
M
a (large acceleration)
(small acceleration)
The acceleration of the cannonball is great
compared to the acceleration of the cannon.
 A force exerted on a small mass produces a
greater acceleration than the same force exerted
on a large mass.
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Force and Mass
Extend this idea to a rocket launch.
 Combustion of rocket fuel produces
“expanding gases”. Each molecule of
exhaust gas acts like a tiny molecular
cannonball shot downward.
 In reaction. the rocket recoils from the
molecular cannonballs shot downward and
climbs upward.
 No atmosphere is necessary for this action
& reaction.
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Understanding Lift
Using Newton’s 3rd law, we can
explain how a helicopter, bird,
or plane gets its lifting force.
 The whirling blades of the
copter (or the wings of a bird
or the wings of an airplane)
force air downward; the air
forces the blades (or wings)
upward.
 The upward reaction force is
called lift.
 When lift equals the weight of
an object, the object can
hover; when lift is greater, it
can climb upward.
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Understanding Lift
A plane, though, must continuously push
air downward to remain in the air.
 A continuous supply of air is produced by
the forward motion of the plane.
 The forward motion is supplied by jets or
propellers. They push air backwards and,
in turn, the air pushes the plane forward.
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Check your Understanding
Explain why the acceleration of a recoiling
gun is much less than the acceleration of
the bullet.
 Are rockets able to accelerate in space?
Why or why not?
 A tug-of-war takes place between Ms. P’s
physics class and Mrs. D’s. If the floors
are polished and slippery and if Mrs. D’s
entire class is wearing socks only, who will
win and why?
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7.5 Defining Systems
•Since action-reaction
forces are equal and
opposite, why don’t
they cancel to zero?
•Simply put, the forces
do not cancel because
they are forces on
different systems
(different objects).
7.5 Defining Systems
1
2
•To explain this answer, we must consider the
systems involved.
•There are 2 systems in the diagram above.
7.5 Defining Systems
In system 1, we see a vector (arrow) extending
outside the system. This is a force on the
system, so the system accelerates (Newton’s 2nd
Law).
 The force was provided by the apple, which is
outside the system. Any reaction force (orange
on the apple) is outside the system, so it does
not affect the orange.
 Action and reaction forces do NOT cancel when
either force is outside the system being
considered.
 What if we put a dotted line around the apple
instead of the orange? Our system will be the
apple so . . .
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7.5 Defining Systems
In system 2, since the action/reaction
forces are within the system, they DO
cancel each other. They do not cause the
system to accelerate.
 A force outside the system – FRICTION –
is needed.
 The apple pushes on the floor; the floor
pushes on the apple. The system
accelerates.
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7.5 Defining Systems
Another example: a
baseball has an
immeasurable number of
forces between the atoms
WITHIN the ball.
 These forces hold the ball
together; they DO NOT
cause the ball to
accelerate.
 An external force, like a
bat, is needed to
accelerate the ball.
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7.6 The Horse-Cart Problem
Take a look at the
cartoon on pg.
115 in your test.
 This “problem”
can be thought of
as containing 3
different systems.
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– The cart
– The horse
– The horse and cart
7.6 The Horse-Cart Problem
The cart system: the cart will accelerate because
there is an external force being place on it by
the horse. There is also an external force of
friction but this can be discounted because the
wheels are smooth and shiny.
 The horse system: there is a reaction force from
the cart that is external to this system. It will
therefore act to restrain the horse. However,
the horse also interacts with the ground – the
horse pushes backward and the ground pushes
forward.
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7.6 The Horse-Cart Problem
If the horse in the horse-cart system pushes the
ground with a greater force than it pulls on the
cart, there is a net force on the horse and the
horse-cart system will accelerate.
 The horse-cart system: the pull of the horse on
the cart and the cart on the horse are internal
forces and do not accelerate the system. The
interaction between the horse-cart system and
the ground is responsible for motion.
 Acceleration = force of the push on the ground
mass of the horse and cart
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7.7 Action Equals Reaction
For every
interaction
between things,
there is always a
pair of oppositely
directed forces
that are equal in
strength.
Practice
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List the forces acting on the
– Tractor system
– Elephant system
– Man system
Practice
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Consider the interaction depicted above between foot A,
ball B, and foot C. The three objects interact
simultaneously (at the same time). Identify the two pairs
of action-reaction forces. Use the notation "foot A", "foot
C", and "ball B" in your statements.
Under what conditions would the ball accelerate?
Practice
If A and B have equal
mass, what will
happen when they
push off from each
other?
 Suppose A has a
greater mass than B.
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A
B