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### Course: AP®︎/College Physics 1>Unit 3

Lesson 4: Newton's third law

# Newton's third law review

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).
The swimmer pushes against the pool wall with her feet (${F}_{\text{feet on wall}}$). The wall exerts an equal and opposite force back on the swimmer (${F}_{\text{wall on feet}}$), 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 ${F}_{\text{wall on feet}}$ is an external force on this system and the swimmer moves in the direction of ${F}_{\text{wall on feet}}$.
In contrast, the force ${F}_{\text{feet on wall}}$ acts on the wall and not on the swimmer. Thus, ${F}_{\text{feet on wall}}$ does not directly affect the motion of the system and does not cancel ${F}_{\text{wall on feet}}$. 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 ${F}_{g}=mg$, objects also pull on the Earth.

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.

## Want to join the conversation?

• How is it possible to have unbalanced forces given Newton's 3rd law? If I push down on a table and the table breaks, where's the reaction coming from?
• There is a reaction. When you push on the table, the table is pushing back on you. The table breaks because it is unable to withstand the force you put on it. The fact that the table breaks is irrelevant to the notion that the table exerts a force back on you.

As for your other question: This is a very common misconception. Think about it, Newton's third law deals with an interaction between two objects. This two objects will both experience a force of equal magnitude. But now if you focus on only one object and ignore the other. You will notice that there is only one force acting on that object. Therefore, it will accelerate. Hope this helps!
• Did not understand this: "they do not because they act on different systems. If the swimmer is the system, then F,wall on feet is an external force on this system and the swimmer moves in the direction of F, wall on feet."
• The swimmer is pushing on the wall and the wall is pushing on the swimmer. Therefore, the forces are balanced but not acting on the same object so the net force will not be zero and the objects could move depending on their mass
• If an equal and opposite force is always exerted on the first object by the second object, then how can things ever move. Wouldn't they always have a net force of 0 netwons?
• Why doesn't the wall move?
• The force acting on the person and the wall might be the same, but remember that a=f/m. The swimmer has little enough mass that the acceleration is still a lot, but the wall is so massive that the acceleration is not noticeable. If the swimmer were to push off a very thin wall with nothing behind it, that wall would probably move too.
• The normal force is acting on the object and the Weight-Mg is acting on the ground, then why aren't they an action-reaction pair?
• No, I don't have a question.
• Isn’t “the force exerted on an object” and “the force experienced by the object” the same thing? The definition of action-reaction pair in this article says that the former is the action and the latter is the reaction, but I thought the action would be the force exerted by the object.