Transcript Chapter 6: Newton`s third law of motion – action and

Chapter 6: Newton’s Third Law
of Motion
Action and Reaction
THINK
1.) What is Newton’s first law of motion?
2.) What is Newton’s second law of
motion?
3.) A 30 kg refrigerator is pulled down a
loading ramp with a force of 45 N. Find
the acceleration of the refrigerator.
4.) What is a force?
Newton’s Third Law of Motion
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.
 It doesn’t matter which force we call the
action force and which we call the reaction
force – they are both equal in magnitude and
always opposite in direction.
Newton’s law can be stated in other words:
For every action, there is an equal and
opposite reaction.





A ball is dropped off of a building and falls to the
ground. Earth exerts a force on the ball (the force
of gravity). In turn, the ball exerts a force on earth.
The force that the ball exerts on earth is the same
as the force that earth exerts on the ball.
The ball accelerates toward earth as a result of the
action force. Does this mean that earth
accelerates toward the ball, as a result of the
reaction force? Yes!
Why do we not notice the acceleration of earth
towards the ball?
Recall Newton’s Second Law of Motion: a = F / m
The force acting on the ball and acting on earth is
the same, but the masses are much different.
Because earth has such an enormous mass, the
acceleration is incredibly small, too small for us to
even witness!
Action and Reaction Forces DO
NOT CANCEL
Even though action and reaction forces
are equal in magnitude and opposite in
direction, they do not cancel each other
out.
 Forces that are equal in magnitude and
opposite in direction cancel each other
out only when they are acting on the
same object.

A child pulls a wagon down the street
Force that wagon exerts on child
Force that child exerts on wagon
Friction force
Friction force
Think
1.) What is Newton’s third law of motion?
2.) Identify the action and reaction forces in each
situation:
(a) A tennis ball is hit by a tennis racket
(b) An apple falls to the ground
(c) A car crashes into a guardrail
3.) Suppose a friend who hears about Newton’s
Third Law of Motion 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 you 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?
Draw a diagram for each of the following
situations, just like we did with the child and
the wagon. Label all the horizontal forces
acting on each object.
1.) A person pushing a crate along the sidewalk
2.) A person walking his dog
3.) A child pulls another child on a sled
In each situation, what is the action force?
What is the reaction force?