Review the key concepts and skills for Newton's third law of motion, including how to identify action-reaction pairs.

Key terms

TermMeaning
Action-reaction pairThe force exerted on an object is the action, and the force experienced by the object as a consequence of Newton’s third law is the reaction.

Newton’s third law of motion

Newton’s third law of motion says whenever one body exerts a force on a second body, the second body exerts a force that is equal in magnitude and opposite in direction on the first body.

How to identify action-reaction pairs

We can see Newton’s third law at work by taking a look at how people move. Consider a swimmer pushing off from the side of a pool (see Figure 1 below).
Figure 1. A swimmer pushes on the wall with her feet, which causes the wall to push back on her feet due to Newton's third law. Image credit: Adapted from OpenStax College Physics. Original image from OpenStax, CC BY 4.0
The swimmer pushes against the pool wall with her feet (Ffeet on wallF_\text{feet on wall}). The wall exerts an equal and opposite force back on the swimmer (Fwall on feetF_\text{wall on feet}), causing her to accelerate in the direction opposite to that of her push.
We might think that two equal and opposite forces would cancel, but they do not because they act on different systems. If the swimmer is the system, then Fwall on feetF_\text{wall on feet} is an external force on this system and the swimmer moves in the direction of Fwall on feetF_\text{wall on feet}.
In contrast, the force Ffeet on wallF_\text{feet on wall} acts on the wall and not on the swimmer. Thus, Ffeet on wallF_\text{feet on wall} does not directly affect the motion of the system and does not cancel Fwall on feetF_\text{wall on feet}. Note that the swimmer pushes in the direction opposite to that in which she wishes to move. The reaction to her push is thus in the desired direction.

Common mistakes and misconceptions

  1. People sometimes think force pairs cancel, resulting in no motion. The force pairs do not cancel, however, because they act on different systems. For example, a swimmer pushing off a pool wall (the action) exerts a force on the wall, and the wall also exerts a force (the reaction) on her. To figure out if she accelerates, we only consider the forces on her and then apply Newton’s second law. See the “How-to identify action-reaction pairs” section above for more details.
  2. People sometimes forget that Newton’s third law also applies to gravity. Just as the Earth pulls down on an object with a force Fg=mgF_g = mg, objects also pull on the Earth.

Learn more

For deeper explanations, see our videos introducing Newton's third law and misconceptions about Newton's third law.
To check your understanding and work toward mastering these concepts, check out the exercise on identifying equal and opposite forces.
This article was adapted from the article "Newton’s Third Law of Motion: Symmetry in Forces" from Openstax College Physics.
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