Newton’s Third Law
of Motion
Action
And
Reaction
Introduction
• If you lean over – you
fall
• If you lean over with
your hands against
the wall – you don’t
fall
• Why?
• Because the wall is
pushing on you and
holds you in place
Forces and Interactions
• Force – a push or pull
• Newton saw an
interaction
• Mutual action between
forces
• Hammer and nail
• Hammer moves nail
• Nail halts the hammer
Newton’s Third Law
• Whenever one object exerts a
force on a second object, the
second object exerts an equal
and opposite force on the first
object
Third Law Forces
• Action force –
• Reaction force –
• It doesn’t matter
which is which
• Partners in the
interaction
• Equal and opposite
• Neither can exist
without the other
Examples
• You push on floor, the
floor pushes on you
• Car pushes on road, the
road pushes on the car
• Swimming – you push
water backward, the
water pushes you
forwards
• More friction – more force
Questions
• Does a stick of
dynamite contain
force?
• No, force is an
interaction between
two objects. An
object may be capable
of creating a force,
but it can’t possess it.
Dynamite possesses
ENERGY
Questions
• A car accelerates along the
road. Strictly speaking, what is
the force that moves the car?
• The road pushes the car. It
provides a horizontal force.
Identifying Action
and Reaction
• Sometimes it is hard to find the
action/reaction pairs
• What are the forces on a falling
boulder?
• Gravity?
• Weight?
• Ground?
• No – none of these
Recipe for finding forces
1. Identify the interaction
2. Action: Object A exerts force on
object B
3. Reaction: Object B exerts force on
object A
•
Falling boulder – Earth exerts force
on boulder, Boulder exerts force on
Earth
Action and Reaction on
Different Masses
• Boulder-Earth interaction –
the forces are EQUAL
• Does the earth fall into the
boulder?
• Yes, but not as far
• The reactions are equal and
opposite
• Because Earth is so large,
we can’t sense its very
small acceleration
Question
• We know the Earth
pulls on the moon.
Does the moon also
pull on the earth?
If so, which pull is
stronger?
• Yes, the Earth and
moon both pull on
each other. Both
pulls are equal and
opposite.
Cannon/Cannon ball
• When fired, the cannon ball
exerts a force on the cannon
• The cannon recoils or “kicks”
• Why doesn’t the cannon
move as fast as the cannon
ball?
• A given force exerted on a
small mass will result in a
greater acceleration than on
a larger mass
Rocket propulsion
• Air escaping from a
balloon
• The balloon accelerates
the opposite direction as
the escaping air.
• Rocket – each molecule
of escaping gas acts like
the cannon ball
Rocket in air and space
• Once though the rocket needed
air to “push” against.
• Actually, rockets work better
without air drag (in space)
Momentum
• How can a karate
expert break a
stack of bricks?
• Why does it hurt
more to fall on
concrete than a
wooden floor?
• Why do you follow
through in golf,
baseball or
bowling?
Momentum
• Momentum = mass x velocity
•p=mxv
• How much momentum does a
400 kg bike moving 40 m/s
have?
• p= mv = (400 kg) ( 40 m/s) =
16,000 kgm/s
Helicopters
• Helicopters – blades
push air down. Air
pushes chopper up.
• Lift – upward reaction
force
• When lift equals
weight of chopper, it
can hover
• When lift is greater,
the chopper rises
Birds and Airplanes
• Fly due to action/reaction
forces
• Wings of bird reflect air
downward. The air then pushes
upward
• Airplanes must have a
continuous flow of air in order
to have lift.
• The engines push air back. The air
Do Action and Reaction
Forces Cancel ?
• Why do the equal
forces not cancel to
zero
• How can there be
acceleration?
• There are MANY equal
and opposite forces in
a system.
• If two people kick the
ball at the same time –
together they cancel
out
Question
• Suppose a friend who hears about
Newton’s law say that you can’t
move a football by kicking it because
the reaction force by the kicked ball
would be equal and opposite to your
kicking force. The net force would
be zero, so no matter how hard you
kick, the ball won’t move! What do
you say to your friend?
Answer
• Obviously kicking the ball will
accelerate it. Your kick acts on
the ball. It accelerates the ball.
The reaction force acts on your
foot – it decelerates your foot.
The horse-cart problem
• If a horse pulls on a
cart, doesn’t the cart
pull back equally?
• Three views
• Farmer – get the cart to
market
• Horse
• Horse-cart system
Farmer
• Only concerned about force
exerted on the cart
• Force on cart/mass of cart =
acceleration
• Doesn’t care about the reaction
on the horse
Horse system
• Opposite reaction force by the
cart restrains the horse
• So how can the horse move
forward?
• The horse must push on the
GROUND – which pushes back.
Horse-Cart system
• The pull of the horse on the cart
and the reaction of the cart on
the horse are internal forces.
• They contribute nothing to the
acceleration
• The acceleration comes from
the horse-cart system and the
ground
Stalled car
• You can’t move the car by sitting in your
seat and pushing against the dashboard
• You must get outside and push against the
ground.
• Don’t worry if you get confused, Newton
struggled with his law too.
Action Equal Reaction
• What if you hit a wall?
• It hits back
• You can’t hit it harder
than it can hit you back
• What if you hit paper?
• It can not hit back as
hard
Conclusion
• Push the world hard – it
pushes back hard
• Push the world gentle –
it pushes back gentle
• You can not touch
without being
touched!!!