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

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  1. 0:07 Newton's Third Law of Motion
  2. 1:17 Two Forces for Every Object
  3. 1:51 Implications
  4. 3:03 Machines
  5. 3:59 Nature
  6. 4:43 Space
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Taught by

John Simmons

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

This lesson describes Netwon'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

When you sit in a chair, the force of gravity is balanced by the force of the chair pushing up
Forces Balanced
Are you sitting down for this? If not, you better take a seat. Notice that you're not falling through the chair or falling through the floor. That's because the forces acting on you are balanced. The force of gravity pulling down on you is balanced by the force of your chair pushing up. What's a force? A force is a push or a pull of an object that results from interaction with another object. Some forces result from contact - for example, the normal force of your body on the chair and the friction between the wheels of a car and the road. Other forces are the result of interactions from a distance - for example, gravity, electrical, and magnetic forces.

When any two objects interact with each other, whether it's direct contact or at a distance, they exert forces upon each other. As you sit in your chair, your body acts on the chair with one force, and the chair reacts on your body with yet another force. This is an example of Newton's third law in action. Newton's third law states that for every action there is an equal and opposite reaction.

Two Forces for Every Object

According to Newton's law, interacting objects experience two forces: action and reaction. The size of the forces acting on one object equals the size of the forces acting on the second object. Additionally, the direction of the force on one object is opposite to the direction of the force on the second object. Let's consider a man attempting to lift a really heavy weight. If the man is able to pull up with a force of 100 lbs, then the weights in turn will pull down with a force of 100 lbs.

If a man pulls up a weight with a force of 100 lbs, the weight pulls down with a force of 100 lbs
Third Law

Implications of Newton's Third Law

There are plenty of implications of Newton's third law of motion. Imagine yourself strolling through the woods. Your foot acts on the ground, pushing it backwards. The ground then reacts on your foot, pushing you forward. The force of your leg on the earth is equal to the force of the earth pushing back on you. Additionally, the force of the leg is opposite to the direction of the earth pushing back, thus propelling you forward. Any change in motion is termed acceleration. Therefore, you actually accelerate as you take that first step.

What about the earth? Does the earth accelerate? Any acceleration of the earth will go unnoticed because the earth's mass is so large in comparison to your mass. Acceleration is directly proportional to the force and inversely proportional to the mass of the object. Since the forces are equal and the masses are so incredibly different, the acceleration is incredibly different as well. Now let's talk about how Newton's third law behaves first in machines, and then in nature, and finally in space.

Newton's Law and Machines

First up, machines. Consider a moving car. The spinning wheels push backwards on the road. The road responds with an equal and opposite force, pushing the car forward. Just like in our previous walking example, any movement of the earth is negligible since the earth is so large in comparison with the car.

The recoil that occurs after shooting a gun is due to the third law of motion
Gun Recoil
Guns, like cars, are machines, and they utilize Newton's third law of motion as well. If you've ever shot a gun, you have experienced the recoil force when the gun is fired. As the gunpowder explodes within the gun, the gases expand and push forward on the bullet, thus propelling it out of the gun. The bullet, in turn, pushes back on the gun, resulting in that recoil that you feel. The force acting on the bullet and the gun are equal and opposite.

Newton's Law and Nature

How about the birds of the air and the fish in the sea? They can both be examples of Newton's third law in practice as well. Let's consider first the bird. The bird's wings push down on the air and then, in turn, the air pushes up on the wings with the same amount of force. While the wing pushes down on the air, the air pushes up on the wing, and that gives the bird flight. What about the fish? A fish's tail moves side-to-side as it swims forward. The tail pushes on the water. The water, in turn, reacts with the same force on the tail, pushing the fish forward.

Newton's Law in Space

How does Newton's law work in space? If space is void of matter, then what does a spaceship push off to propel itself forward? That's a good question. As it turns out, the exploding fuel pushes off on its own exhaust fumes. The fumes, in turn, will push back on the rocket with an equal and opposite force, propelling the rocket forward into space.

A rocket can move through space because the exhaust fumes push back on the rocket to propel it
How Rocket Moves Through Space

Lesson Summary

In summary, Newton's third law states that for every action there is an equal and opposite reaction. These actions are what we call forces, simply a push or a pull on an object that results from interaction with another object. Objects can interact with each other by direct contact or at a distance. The interacting objects experience two forces that are equal in magnitude and opposite in direction. No matter the size of the interacting objects, the forces are always equal and opposite. These forces are responsible for acceleration, which is a change in motion. Newton's law applies to objects on Earth and in space.

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