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4.3 Newton’s Third Law of Motion
Warm Up
1. Consider the following situations. Draw a sketch of each situation and, on each diagram, indicate the forces
being described. Share your results with the class.
(a) If you wish to climb stairs, in which (c) If you are rowing a boat, which way must your
direction do you push? Which way to you oars push if the boat is to move forward? Which
move? way to do you move?
(b) If you wish to swim forward, in which (d) When a car is moving forward, in which
direction must your arms push? Which direction do the wheels push on the road? Which
way do you move? way to do you move?
Action and Reaction
Forces
Isaac Newton observed that whenever forces exist between two bodies, the forces are
mutual. If one body pushes on another, the other body exerts an equal force on the first
body, but in the opposite direction. To do push-ups, for example, you push down on the
floor. The floor exerts an equal force up on you. The floor lifts you up! Earth exerts a force
of gravity on the Moon. What evidence is there that the Moon exerts an equal force on
Earth?
Newton studied situations involving forces between pairs of bodies and he stated
his conclusions about mutual forces between pairs of bodies as his third law. Newton’s
third law of motion is also called the law of action and reaction.
If two bodies interact, the force the first body exerts on the second body will equal
the force the second body exerts on the first body. The two forces will be opposite in
direction and will act simultaneously over the same interval of time.
176 Chapter 4 Newton’s Laws of Motion — DRAFT © Edvantage Interactive 2011 ISBN 978-0-9864778-3-6