Download Chapter 7: Newton`s Third Law of Motion – Action and Reaction1

Chapter 7: Newton’s Third Law of Motion – Action and Reaction1
Forces and Interactions
Force: a push or a pull
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A force is always part of a mutual action that
involves another force.
o mutual action: Interaction between
one thing and another
Forces occur in pairs
o For all cases of contact, there is a
twoness – contact requires 2 objects
Newton’s third law: for every action force there
is an equal and opposite reaction force.
Name the interaction here.
Hammer and nail:
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No. Force is not something an object has, like
mass. Force is always part of a mutual action
that involves another force. Dynamite has the
capability of exerting a force on another object,
but does not possess force as a thing in itself.
Hammer exerts a force on
the nail and drives it into
the board
Nail exerts a force on the
hammer to cause it to
stop
Newton’s Third Law of Motion
Newton’s third law of motion involves two forces: the action force and the reaction force
It doesn’t matter which force is called the action force and which is called the reaction
force.
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Both forces are partners in a single interaction
Neither force exists without the other
Equal in strength and opposite in direction
Chapter 7: Newton’s Third Law of Motion – Action and Reaction2
Let’s talk interactions!
Walking: you push against the floor, the floor pushes
against you.
Driving: tires push against the road, the road pushes
against the tires.
Name the interactions
here.
Swimming: you push against the water, the water pushes
against you.
Walking on ice? Friction is minimal, so you may not be able
to exert the action force. If so, there is
no reaction force.
The second object exerts an equal and opposite force
on the first object! Newton’s third law of motion.
Identifying Action and Reaction
Name the reaction force if a boulder is
falling to the ground.
How to identify action and reaction forces:

Identify the interacting objects:
Action: Object A exerts a force on
object B.
Reaction: Object B exerts a force on
object A.
Chapter 7: Newton’s Third Law of Motion – Action and Reaction3
Action and Reaction on Different Masses
The boulder pulls up on the Earth with as much force as
Earth pulls down on the boulder.
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Boulder falls to Earth
Earth falls to boulder!
Forces are equal, but masses are radically different!
Newton’s 2nd law states that acceleration is proportional to
force, but inversely proportional to mass.

Earth’s huge mass makes the acceleration upward
negligible.
When the cannon is fired, there is an interaction between
the cannon and the cannonball.
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Force exerted on the cannon is equal and opposite
to the force exerted on the cannonball.
Cannon recoils
Why does the cannonball move so fast compared to the
cannon?
A given force exerted on a small mass
produces a greater acceleration than the
same force exerted on a large mass.
Balloon accelerates as the air comes
out.
Rockets operate on the same
principle – the rocket continually
recoils from the exhaust gases
ejected from the engine.


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Rocket is not pushing against
the atmosphere
Rocket is recoiling from the
“molecular cannonballs” it
fires out the back, and climbs
upward.
Rockets actually work better in
outer space due to the lack of
air resistance
Chapter 7: Newton’s Third Law of Motion – Action and Reaction4
Lift
Helicopters’ blades force air particles down; the air forces the blades upward.
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Upward reaction force is called “lift.”
When lift equals the weight of the helicopter, the helicopter hovers in midair.
When lift is greater than weight, the helicopter is able to climb.
Birds’ wings deflect air downward; air pushes the birds upward.
Airplane wings also have a shape to deflect oncoming air downward, which provides
lift.
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Airplanes must push air downward to produce lift.
Jet engines or propellers push air backward, so the air pushes the airplane
forward.
A given force exerted on a small mass produces a greater acceleration
than the same force exerted on a large mass.
Defining Systems
Since action and reaction forces are equal and opposite, why don’t they cancel to zero?
You have to consider the system involved.
Dashed line defines the system.
Red vector arrow points outside the
system and represents an external force.
System accelerates (Newton’s 2nd law)
Apple provides the force. Apple is
outside the orange system.
Orange exerts a force on apple
(another system).
Action and reaction forces do not cancel each other when either of the forces is external
to the system being considered.
Chapter 7: Newton’s Third Law of Motion – Action and Reaction5
Force pair is internal to the orangeapple system.
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These forces do cancel.
These forces do not accelerate
the system.
Force external to system is needed to
accelerate the system.
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Apple pushes against the floor
Floor pushes against the apple
(friction)
Orange-apple system accelerates
Action-reaction forces internal to the system cancel out and play no role in accelerating
that system.
Let’s play some football!
One interaction between the foot and the football:

The ball accelerates.
Two kicks act on the same ball (2 interactions
occurring):
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If both kicks are simultaneous, equal, and
opposite, the net force on the ball is zero.
Opposing forces act on the same object, not
different objects, so they do not make up an
action-reaction pair.
Suppose a friend who hears about
newton’s third law says 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?
Action and reaction forces do not cancel each other
when either of the forces is external to the system
being considered.
Chapter 7: Newton’s Third Law of Motion – Action and Reaction6
Action Equals Reaction
Can you hit a falling piece of paper
with 50 pounds of force? Why or
why not?
You can’t because the paper can’t
hit back on you with 50 pounds of
force.

A 50-lb interaction between
your fist and the paper is
not possible.
For every interaction between things, there
is always a pair of oppositely directed forces
that are equal in strength.