Newton’s Third Law
Action/Reaction Force Pairs
Review of Newton’s Laws

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Newton’s First Law - tells you what happens to
an object if the net force on it is zero.
Newton’s Second Law - tells you what
happens to an object if the net force on it is not
zero.
What could Newton's Third Law possibly tell
you?
Newton’s Third Law

In a sense, Newton's First and Second
Laws tell you what forces do. Newton's
Third Law tells you what forces are.
Newton's Third Law is often stated:
For every action there is an equal and opposite
reaction.
"action...reaction" means that
forces always occur in pairs.

Forces are interactions between objects.
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Single, isolated forces never happen.
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The two forces involved are called the "action
force" and the "reaction force.“

Either force in an interaction can be the
"action" force or the "reaction" force. See
Identifying Action and Reaction Forces.
Difficulties with Newton's Third
Law:
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
"If A pushes B, then B pushes A with an
equal and opposite force. If these forces
are equal and opposite, they cancel,
producing a net force of zero.
This means that neither object can
accelerate, which means that Newton's
Laws predict that nothing can ever
move."
Only the forces that act on the
same object can cancel!

Forces that act on
different objects don't
cancel.

The Force on the
wall is equal and
opposite to the force
on the ball, BUT:
The Solution:


These forces DO NOT CANCEL
because they influence the motion of
different objects.
The force that the wall exerts on the ball
influences the ball’s motion, and the
force that the ball exerts on the wall
influences the wall’s motion.
Why does the skater move?
1.
2.
3.
What are the action/reaction forces?
F wall and F skater
What objects are each of the forces acting on?
F wall – on the skater
F skater – on the wall
Are the force pairs equal and opposite?
Yes, but they are acting on different
objects so they do not cancel.
4.
Why does the skater move but the wall does
not?
the wall is too massive to move with the force
exerted by the skater, but the wall exerts a
force large enough to move the skater.
Ropes and Tension

• Tension is always
a pulling force.
This means it is
directed away from
an object.

Fnet = FT - Fg
Practice

You are helping to repair a roof by
loading equipment in a bucket that
workers hoist to the rooftop. If the rope
is guaranteed not to break as long as the
tension does not exceed 450 N and you
fill the bucket until it has a mass of 42
Kg. What is the greatest acceleration
that the workers can give the bucket as
they pull it to the roof?
Solution
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Fnet = FT –Fg
ma = FT – mg
(42 kg)a = FT – (42 kg)(9.8 m/s2)
(42 kg) a = 450 N - 412 N
a = 450 N – 412 N
= 0.91 m/s2
42 kg