Sec. 6.3
Interaction Forces
Newton’s Third Law
► Objectives
 Explain the meaning of interaction pairs (third
law pairs) of forces and how they are related by
Newton’
Newton’s third law
 List the four fundamental forces and illustrate
the environment in which each can be
observed.
 Explain the tension in ropes and strings in terms
of Newton’
Newton’s third law
Newton’s 3rd Law
Newton’s laws of motion
For every action force there is
an equal and opposite
reaction force
1. The law of inertia. An object in motion
remains in motion with constant velocity if the net
force on the object is zero.
2. Force and acceleration. If the net force acting
on an object of mass m is F, then the acceleration
of the object is a = F/m. Or, F = ma.
(You cannot touch without being touched)
3. Action and reaction. For every action there is
an equal but opposite reaction.
Action means force.
Interaction forces
(‘Third Law Pairs’
Pairs’)
Action-reaction Pair
If object A exerts a force on object B, then object B
exerts an equal and opposite force on object A. The pair
of forces (due to one interaction), is called an
action/reaction pair.
► Two
forces that are in opposite directions and
have equal magnitude
► They
never act on the same object!
► Newton’
Newton’s
► FA on B
Third Law—
Law—all forces come in pairs
= -FB on A
NOTA BENE:
The action/reaction pair will never appear in the same free body diagram.
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Newton’s third law
For every action there is an
equal but opposite reaction.
Whenever one object exerts a force on
another object, the second object exerts an
equal but opposite force on the first object.
force of A on B
force of B on A
Forces always occur like this, in pairs.
We will see that this is very hard to accept! It is just not
common sense. That is why it took a great genius like
Newton to figure it out.
Action: tire pushes on road
Reaction: road pushes on tire
When you walk or run, what forces occur?
Recoil force
• At constant velocity the horizontal force is 0 and you
continue to move because of inertia.
m
a = F/
Force accelerating bullet
a = F/m
• To accelerate, you push backward against the floor; the
reaction force, which is a friction force exerted by the
floor on your foot, pushes you forward.
This reaction force may be hard to
visualize, but imagine what would
happen if you were on a frictionless
surface – can’t accelerate!
A bug and a car collide. Which
experiences the greater force?
• (a) bug
• (b) car
• (c) neither, they both experience the same
magnitude of force
Consider hitting a baseball with a
bat. If we call the force applied to
the ball by the bat the action force,
identify the reaction force.
(a) the force applied to the bat by the hands
(b) the force applied to the bat by the ball
(c) the force the ball carries with it in flight
(d) the centrifugal force in the swing
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A puzzle:
The truck pulls to the right. According to Newton’s
third law, the car pulls to the left with an equal
force. So how can they start moving, or accelerate?
Which vehicle exerts a greater force
― the tow truck or the car?
Resolution: Consider each part separately, and
don’t forget that other forces are also acting.
Playing catch with
a medicine ball
Example – A collision
A
B
A throws the ball and B catches it.
 four forces
When A throws the ball he exerts a force on the ball
(toward the right) and the ball exerts a force on him
so he recoils (toward the left).
► Newton’s third law for the throw
When B catches the ball he exerts a force on the ball
(toward the left to stop it) and the ball exerts a force on
him so he is knocked back (toward the right).
► Newton’s third law for the catch
A puzzle : Tug of War
String tension
String tension
The force exerted by the
ball on the toe (reaction) is
equal to the force exerted
by the toe on the ball.
Really hard to accept!
Contact force
Contact force
Which team will end up in the puddle?
But aren’t the forces equal but opposite !?
Resolution: Don’t forget that there are other forces acting.
Each team exerts a force on the Earth, so the Earth exerts a
force on the team (3rd law!). The net force on either team is
toward the left.
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A puzzle
Horse and Cart
The horse pulls the cart with a force A (to the right).
According to Newton, the cart pulls the horse with a
force –A (to the left).
So how can they start moving, or accelerate?
Resolution: Consider each part separately, and don’t
forget that there are other forces acting.
A small car is
pushing a larger
truck that has a dead
battery. The mass of
the truck is larger
than the mass of the
car. Which of the following statements is true?
A small car is
pushing a larger
truck that has a dead
battery. The mass of
the truck is larger
than the mass of the
car. Which of the following statements is true?
A. The truck exerts a larger force on the car than the car exerts
on the truck.
B. The truck exerts a force on the car but the car doesn’t exert a
force on the truck.
C. The car exerts a force on the truck but the truck doesn’t exert
a force on the car.
D. The car exerts a larger force on the truck than the truck
exerts on the car.
E. The car exerts the same amount of force on the truck as the
truck exerts on the car.
Newton’s Third Law
A. The truck exerts a larger force on the car than the car exerts
on the truck.
B. The truck exerts a force on the car but the car doesn’t exert a
force on the truck.
C. The car exerts a force on the truck but the truck doesn’t exert
a force on the car.
D. The car exerts a larger force on the truck than the truck
exerts on the car.
E. The car exerts the same amount of force on the truck as
the truck exerts on the car.
The four fundamental forces
Tactics: Analyzing interacting
objects
• Gravity
• Electromagnetic forces
• Strong nuclear force
• Weak nuclear force
All the fundamental interactions obey conservation
of momentum (verified by experiments), which is
equivalent to Newton’s third law.
Nature appears to be complex; but
beneath the surface, nature is simple.
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