Isaac Newton was a legendary scientist and inventor of the 17th century. He laid the foundation of modern mechanics with this Three Laws of Motion. These remain the basis of physics even today (if you’d like to learn more, this course can give you an overview of physics essentials). In today’s post we’ll explain Netown’s Third Law of Motion, with some real world examples. You may want to look up his First and Second laws here [Note to editor: Please insert links to the blog posts for Newtons’  1st and 2nd law here. They haven’t been published as of my writing this post].

According to Newton’s Third Law of Motion  “Every action has an equal and opposite reaction”. This means in every interaction, there are always two forces at play on the body. The magnitude of the two forces is exactly the same. However, the directions are opposite. Thus, they balance each other. Let’s take an example. When you sit on a chair, your body exerts a downward force on the surface of the chair, while the the chair exerts an equal and upward force on your body. As both are balanced, the chair and you stay in the same position. This is an example of Newton’s third law in which the two forces are known as action and reaction forces.

## What is Force?

Force is defined as a push or pull upon an object which results from its interaction with another object. Whenever two objects interact force is exerted on each of the objects. When there is no interaction, the force on each object ceases. Forces can be classified into two types – Contact forces and action-at-a-distance forces.

1. Contact forces

In this case the objects that interact with one another are physically in contact. These forces include applied forces, frictional forces, tensional forces and more.

1. Action-at-a-distance forces

Here unlike contact forces the objects that interact with one another are not in physical contact. Examples of these forces are gravity, magnetic, electric and even inter-particle forces. You can get an insight into such forces with this course on Quantum Physics.

We will take a closer look at the Third Law of Motion using examples.

## Example 1: How Fish Swim

Have you ever wondered how a fish swims in the water? A fish pushes water backward by using its fins. What happens then is the water pushes the fish forward with same force as the fish had exerted on the water. Hence, action-reaction force pairs enable the fish to swim. The same technique is applied by human swimmers. Here they use their arms to push the water backward which propels them to move forward. This course can help you learn to swim like a Pro using these very principles.

## Example 2: Impact of a Moving Football

Everyone loves football, right? Football is always a great way to bond – whether you’re watching it at the stadium or on TV (if you’re from the other side of the globe and want to get familiar with American football, this course tells you the terms and what to watch out for, so can join your American pals in the fun). Say you are watching a football match at a  stadium. Suddenly a football comes in your direction. You manage to push the ball away from you with your hands. However, the impact of the ball causes pain in your hands. Why does this happen? This is because when two objects interact with each other, each exerts equal and opposite forces on each other.

## Example 3: How Birds Fly

The wings of a bird force air downwards, these results in the air applying force of the same magnitude upwards. This pushes the bird  upwards into the air. The same concept applies to planes and helicopters. Never thought such a basic principle is behind that cushy air travel! You can also learn to fly – just sign up for this course.

## Example 4: Automobiles Propulsion

This principle can also help us understand how automobiles move on the road. When a car is started its wheels spin and grip the road. This results in a force that pushes the road backwards. As the result the road exerts a force equal in size of the wheels but in the opposite direction, i.e. forward. This results in the car moving ahead on the road. In case the surface of the road was ice, the wheels wouldn’t be able to grip road properly. Hence, it would be difficult to exert a force. This is why automobiles find it difficult to move ahead on icy surface and end up skidding. Yes, Newton’s humble Third Law at play here as well.

## Example 5: Rocket Travel Made Possible

In the past space travel was considered impossible. This was because space was effectively vacuum.  What can a rocket push against in vacuum?  So what do present day rockets do? When a rocket is started in space, hot gases are released in downward direction of the rocket. This results in an equal size upward force on the rocket that pushes the rocket ahead. This is a classic example of Newton’s Third Law.

## Example 6: Gravitational Force

Consider the case of tossing a ball upward. Due to the gravitational force exerted by the earth the ball follows the projectile trajectory and comes down towards the earth. When it hits the earth, the ball applies the same force but in opposite direction to the earth. The question is why the earth doesn’t move towards the ball. It is because the mass of the earth is much bigger than that of the ball. Hence, the acceleration applied on the ball by the earth is negligible.

## Example 7: Push Force Involving Different Masses

Take a look at this situation. Two teenagers are standing on the ice facing each other. Assume that there is zero friction produced by the ice surface. The weight of one of the teenagers is more than the other. Now they push each other in the opposite direction to each other. Here the question is who will move away with higher speed. As the mass of one of the teenagers is greater, his speed will be lower. So the lighter teenager will move away faster.

## Example 8: Hitting the Wall with Your Fist

Frustrated and hitting the wall with your fist? Well, Newton isn’t far away. Here there are two forces, your force and an equal and opposite force applied by the wall on the fist. Hence, harder you hit the wall the more force is exerted on your fist by the wall. This is why you get hurt more.

## Example 9: How Swimmers Start off in a Race

In swimming races, while starting the swimmer uses his feet to push off the wall. The more force he applies on the wall, the faster will be his movement in water. The reason being the wall exerts the same force on the swimmer as he applies on the wall, but in the opposite direction.

## Example 10: How Sea Animals Move in Water

Let’s take an example of sea animals such as octopus. They suck in seawater and eject it through a siphon. As the octopus applies the force on the water, the water also exerts similar force but in the opposite direction. Sound familiar? Yes, it’s the Third Law of Motion at play again.

We hope these examples helped show you how the laws of Physics are at play all around us. They may sound basic, but they pack a lot.

Page Last Updated: February 2020