Transcript Chapter 7 Newton’s third law of motion – Action and Reaction

For every force, there is an equal and opposite force.
CHAPTER 7 NEWTON’S THIRD
LAW OF MOTION – ACTION AND
REACTION
For every force, there is an
equal and opposite force.
 Define force as part of an interaction (7.1)
 State Newton’s third law of motion (7.2)
 Describe how to identify a pair of action reaction
forces (7.3)
 Explain why the accelerations caused by an
action force and by a reaction force do not have
to be equal (7.4)
 Explain why an action force is not canceled by a
reaction force(7.5)
 Explain how a horse-cart system accelerates (7.6)
 Explain what must occur in every action between
things (7.7)
7.1 Forces and Interactions
 A force is always part of a mutual action that
involves another force
 The interaction that drives the nail is the same
that stops the hammer
 A mutual action is an interaction
 Force is an interaction between
one object and another
7.2 Newtons Third Law
 Newtons third law states that whenever one
object exerts a force on a second object, the
second object exerts an equal and opposite
force on the first object
 When the girl jumps to shore, the boat moves
backward
7.3 Identifying Action and Reaction
 To identify a pair of action-reaction forces,
first identify the interacting objects A and B,
and if the action is A on B, the reaction is B on
A
 Action: object A exerts a force on object B
 Reaction: object B exerts a force on object A
7.4 Action and Reaction on
Different Masses
 Apply newtons third law to tug-of-war. If the
action is you pulling on the rope, is the
reaction force the ground pushing back on
you or your opponent pulling back on the
rope?
 Neither the reaction force is the rope pulling back
on you, A on B; B on A
7.4 – continued
 A given force exerted on a small mass produces a
greater acceleration than the same force exerted on
a large mass
 The larger the letter for the variable, the larger the
magnitude that variable has
a
 F
m
=
F
m
=a
 An action -reaction force does not need a medium to
occur, these forces can occur in a vacuum – like outer
space
 When the engines of an airplane push the air
backwards, the air pushes the plane forward and the
shape of the wings force the air down to create lift
7.5 Defining Systems
 Action and reaction forces do not cancel each
other when either of the forces is external to
the system being considered
 So if two people with the same mass push off
each other on ice, they will move backwards
the same distance
7.6 The Horse-Cart Problem
 The forces don’t actually equal zero; it depends
on perspective.
 We need to look at the different systems involved
in the problem
 3 different systems
 Horse
 Cart
 Horse and cart
 The net force on the cart divided by the mass of
the cart will produce an acceleration
7.6 The Horse-Cart continued
 Looking at the horse, the cart does restrain
the horse but the horse can push on the
ground and overcome the force of the cart
 if the horse and cart were in equilibrium then
as the horse walks the cart follows.
 You can’t get a car moving by sitting in the
car and pushing on the dash board you must
get out and push on the ground and the car.
Force
By
On
Direction
Affects the
Motion of
Horse pulls
wagon
Horse
Wagon
Right
Wagon
Wagon
Horse
Left
Horse
Horse
Ground
Left
Ground
Ground
Horse
Right
Horse
Ground
Wagon
Left
Wagon
Wagon
Ground
Right
Ground
Horse pushes
Ground
Friction
Comments
Action/Reaction
Action/Reaction
Action/Reaction
Help on figuring out actionreaction forces
 Difficulties that occur with the action-reaction
situations usually stem from failing to clearly
identify the system in question.
 Basically, if you want to know the effect of a force
or forces on something, call that something your
system.
Define your system by a real or imaginary dotted line
around that something.
2. Restrict your attention to the external forces that
originate outside the dotted line and act on the
system, and NOT to the forces that the system may
exert on things external to the dotted line
1.
7.7 Action Equals Reaction
 For every interaction between things, there is
always a pair of oppositely directed forces
that are equal in strength