Newton's Third Law of Motion: Examples of the Relationship Between Two Forces

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  • 0:04 Newton's Third Law of Motion
  • 0:46 Identifying Action &…
  • 1:51 Equal & Opposite
  • 3:51 Lesson Summary
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Lesson Transcript
Instructor: John Simmons

John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.

This lesson describes Newton's third law of motion. Examples are provided to illustrate how interacting objects experience forces. The lesson explains how objects accelerate as a result of force. Applications of Newton's third law are illustrated in nature, machines, and space.

Newton's Third Law of Motion

Are you sitting down for this? If not, take a seat. You might notice that you're not falling through the chair or floor. That's because the forces acting on you are balanced. A force is a push or a pull in a given direction. The force of gravity pulling you down against your chair is balanced by the force of the chair pushing up against you.

The forces acting on you as you sit in your chair are just one example of Newton's third law, which states that for every action force there is an equal and opposite reaction force. In fact, all forces come in pairs. No force exists in isolation. That's one of the fundamental symmetries of our universe.

Identifying Action & Reaction Forces

Suppose you're standing on the ground. The force of gravity pulls you down towards the earth, which presses your feet against the ground. Let's call that the action force - the force of your feet against the earth. The reaction force is the force of the earth pushing against your feet.

Now imagine that instead of standing, you decide to jump.

Action: You push your feet down against the ground.
Reaction: The ground pushes back up against your feet.

When you take a step forward, your feet are pushing back against the ground; it is only because the ground pushes back against your feet in the forward direction that you're able to move forward.

Now, on to some trickier examples of Newton's third law:

How does a rocket move in space where there's nothing to push off of?

A rocket is basically a controlled explosion. As the rocket fuel burns, it quickly expands outwards in every direction, including against the rocket itself. Here's where Newton's third law comes in.

Action: The expanding gas particles push against the rocket.
Reaction: The rocket pushes against the expanding gas particles.

Equal and Opposite

All of the examples we've discussed so far have involved direct contact between two objects, so let's expand our horizons a bit.

You probably know that the moon orbits the Earth because it feels the tug of Earth's gravity. Let's call the action force the force of the Earth's gravity pulling on the moon. Where's the reaction force? Well, since the moon has mass, it also has its own gravitational field. The moon actually pulls on the Earth with the same force that the Earth pulls on the moon.

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