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## Class 11 Physics (India)

### Unit 9: 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).
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 (F, start subscript, start text, f, e, e, t, space, o, n, space, w, a, l, l, end text, end subscript). The wall exerts an equal and opposite force back on the swimmer (F, start subscript, start text, w, a, l, l, space, o, n, space, f, e, e, t, end text, end subscript), 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, start subscript, start text, w, a, l, l, space, o, n, space, f, e, e, t, end text, end subscript is an external force on this system and the swimmer moves in the direction of F, start subscript, start text, w, a, l, l, space, o, n, space, f, e, e, t, end text, end subscript.
In contrast, the force F, start subscript, start text, f, e, e, t, space, o, n, space, w, a, l, l, end text, end subscript acts on the wall and not on the swimmer. Thus, F, start subscript, start text, f, e, e, t, space, o, n, space, w, a, l, l, end text, end subscript does not directly affect the motion of the system and does not cancel F, start subscript, start text, w, a, l, l, space, o, n, space, f, e, e, t, end text, end subscript. 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, start subscript, g, end subscript, equals, m, g, 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?

• 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
• 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!
• 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?
• 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?
• So all of the planets in our solar system are acting on the sun, it is just so massive that we can't measure its movement?
• 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.
• I think the concept of which force is the action and which is the reaction just depends on the perspective that you view it from. If object X exerts a force on object Y, then Y reacts by exerting a force that is equal in magnitude on object X. But since the forces pretty much act at the same time, you could also say that Y exerts a force and X reacts to it.
I also thought the action was considered to be the force exerted by the object :) , but I guess the distinction between action/reaction doesn't matter too much in most cases.
(1 vote)
• If Newtons law is true, then wouldn't the water exert a force on the swimmer as well? How is she able to move?
(1 vote)
• Oooh! Good question! Umm... I would just say that the movement of the swimmers arms and even other actions can override the force of the water. Also, in the image in the overview, it shows how she is starting of with a leading force by pushing against the wall. Therefore, I infered that these could be the reasons.