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Newton’s Third Law
Chapter 6
Forces and Interactions
• Forces result from the interactions
between objects. Examples are:
– gravitational attraction between the earth and
objects on its surface.
– a push against a wall results in the wall
pushing back.
– the weight of a book and the support or
normal force a table exerts on the book.
– a hammer exerts a force on a nail and the nail
also exerts a force on the hammer.
Newton’s Third Law of Motion
• Whenever an object exerts a force on a
second object, the second object exerts an
equal and opposite force on the first.
OR
• For every action of a force, there is an equal
and opposite reaction force.
FAB   FB A
• Forces always act in pairs, constituting an
interaction: “It takes two to tango!”
Actions and Reactions
More action/reaction
• Action: Object A exerts a force on object B
• Reaction: Object B exerts a force on object A
• Incorrect for situation: Apples falls to ground
– Action: Earth pulls Apple
– Reaction: Apple falls to the ground
• Correct
– Action: Earth pulls Apple
– Reaction: Apple pulls Earth
Action/Reaction on different masses
• If the force on the apple and the Earth
is equal, why does the apple move but
the Earth does not move?
• Acceleration depends on force and
mass. The force are the same but the
masses are not.
• The Earth has a large mass, so it has a
very small acceleration.
• The apple has a small mass and a large
acceleration
Accelerations due to
Action- Reaction Forces
• Although action and reaction forces
are equal in magnitude, the
resulting accelerations depend on
the masses (inertias) of the
interacting objects.
FA B   FB  A
m A a A  mB aB
mB
aA  
aB
ma
A puzzle, the Horse and Cart
• The horse pulls the
cart with a force of
20 N to the right.
• According to
Newton, the cart
pulls the horse
with a force -20 N
to the left.
• So how can they
start moving, or
accelerate?
Action/Reaction and Net Force
• Action/Reaction forces act on
different objects so they do not
cancel
• Net forces act on the same object so
if they are equal and opposite they
can cancel to produce a net force of
zero. The normal force and the
weight both act on the apple.
• In the horse and cart problem,
consider each part separately, and
don’t forget that there are other
forces acting.
Newton’s Third Law Questions
1) While driving, Anna observed a bug striking the windshield of her car.
Obviously, a case of Newton's third law of motion. The bug hit the
windshield and the windshield hit the bug. Which of the two forces is
greater: the force on the bug or the force on the windshield? Explain.
2) A gun recoils when it is fired. The recoil is the result of action-reaction
force pairs. As the gases from the gunpowder explosion expand, the gun
pushes the bullet forwards and the bullet pushes the gun backwards. The
acceleration of the recoiling gun is ... a) greater than the acceleration of
the bullet. b) smaller than the acceleration of the bullet. c) the same size
as the acceleration of the bullet.
3) In the top picture, a physics student is pulling upon a rope which is
attached to a wall. In the bottom picture, the physics student is pulling
upon a rope which is held by the Strongman. In each case, the force scale
reads 500 Newtons. The students pulls…
a) with more force when the rope
is attached to the wall.
b) with more force when the rope
is attached to the Strongman.
c)the same force in each case.
More Newton’s Third Law Questions
1) For the following situations, list action and
reaction forces.
a) A bat hits a ball
b) Bowling ball pushes pin rightwards
c) A satellite orbits the Earth
2) Its is possible for a boxer to hit a wall with a
force of 200 N. Is it also possible for the
same boxer to hit a tissue with that same
amount of force?
3) With what force does an apple weighing 1 N
pull the Earth? Explain.
More review……
Arnold Strongman and Suzie Small pull on
opposite ends of a rope in a tug of war. The
greater force exerted on the rope is by
1. Arnold.
2. Suzie.
3. Neither. The force is the same.
Sources
• Conceptual Physics by Paul Hewitt
• www.physicsclassroom.com
• http://rigel.physics.unr.edu/faculty/phaneuf/c
lassinfo/index100.html