Download Newton`s Third Law of Motion: Action and Reaction

Newton’s Third Law of
Motion: Action and Reaction
Chapter 6
Forces and Interactions
A force is a push or pull
Interaction – a mutual action between
objects where each object exerts an equal
and opposite force on the other
Interacting things exert forces on each
other
Newton’s Third Law
Newton’s third law states:
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 and the
other is called the reaction force (it doesn’t
matter which we call action and which we call
reaction)
Newton’s 3rd law can also be stated as “To every
action there is an equal and opposite reaction.”
In every interaction, the forces always occur in
pairs
When you walk, you are interacting with the
ground; you push against the ground, and the
ground simultaneously pushes against you
Identifying Action and Reaction
Sometimes the identity of the pair of action and
reaction forces in an interaction is not
immediately obvious
First, identify the interaction (one object, A,
interacts with another object, B)
The action and reaction forces can be stated in
the form:
Action: Object A exerts a force on object B
Reaction: Object B exerts a force on object A
Identifying Action and Reaction
Action and Reaction on Different
Masses
The forces are equal in strength and opposite in direction
Remember that Newton’s 2nd law states that acceleration
is proportional to net force, and inversely proportional to
the mass
If we have different masses interacting, the accelerations
on each will be different
If you consider a cannon and a cannonball; the cannon
and cannonball exert equal and opposite forces on each
other, but the cannonball moves much more than the
cannon
Because of Newton’s 2nd law, the mass of the cannon
stops it from accelerating as much as the cannonball
a = Fnet/m
Cannons
F = M(cannon) a(cannon)
F = m(ball) A(ball)
Do Action and Reaction Forces
Cancel?
If action and reaction forces are internal to
a system, they cancel each other and
produce no acceleration of the system
Action and reaction forces do not cancel
each other when either is external to the
system that is being considered
Do Action and Reaction Forces
Cancel?
An apple pulls an orange and the
orange accelerates. The orange
pulls back on the apple, but this
affects the apple and not the
orange.
Both A and C act on B. They
can cancel each other so B does
not accelerate.
The Horse-Cart Problem
Force
Horse pulls Wagon
Horse pushes Ground
Friction
By
On
Direction Affects the Motion of Comments
horse
wagon
right
wagon
wagon
horse
left
horse
horse
ground
left
ground
ground
horse
right
horse
ground
wagon
left
wagon
wagon
ground
right
ground
Action/Reaction Pair
Action/Reaction Pair
Action/Reaction Pair
Action Equals Reaction
You can not hit anything harder than it can
hit you back
If the object is not able to exert a reaction
force equal to your action force, then you
cannot produce an action force that large
Assignment
Read Chapter 6 (pg. 74-82)
Do Ch. 6 Assessment #19-39 (pg. 84-85)
Do Appendix F #1-6 (pg. 667-668)