# What is Newton’s Third Law of Motion?

The horse kicks the man?

Or

The man kicked the horse?

You might be thinking that it’s a simple question, and you’ll say both (man and horse) are kicking each other.

If you are thinking so, you are Right.

But the question is,

How newton’s third law of motion works here?

Let’s make this simple.

See this,

To every action,

there is always an equal and opposite reaction.

That’s it.

The only thing you need to understand in Newton’s 3rd law of motion is,

Force always occurs in pairs.

In short,

Newton’s third law of motion is all about understanding the term force pairs. (Action and Reaction forces)

How? That I’ll show you later.

But before that, read the statement of Newton’s 3rd law of motion.

Newton’s third law of motion states that:

“To every action, there is always an equal and opposite reaction”

Contents

## Newton’s third law examples

In the above statement, do you know the terms “action” and “reaction”?

It is just a pair of forces.

As mentioned above, force always occurs in pairs.

So, these are nothing but action and reaction force pairs.

If you have still not understood, don’t worry.

Let’s make it simple.

Check out some real life examples of Newton’s third law mentioned below from which you will get the exact idea.

And you will surely understand the meaning of the above statement from the below examples.

### #1 A Spring

Just one simple question to ask you guys,

What happens when you press this spring with your hand?

Obviously, the spring is pressed.

But,

Do you know, when you press the spring with your hand, Newton’s third law of motion is applied?

Have you thought, how Newton’s third law works when you press the spring?

And if I ask you to show the action-reaction force pair acting on the spring, then how will you show?

Don’t worry !!

Let me explain it to you in a simple way.

See this figure,

It is clearly seen from the above figure that when you press the spring, two forces are acting.

Your hand applies a downward force on the spring, right? (This is known as action force)

And at the same time,

Spring also applies an upward force on your hand. (This is known as reaction force)

Force always occurs in pairs.

So, together these pairs are known as action-reaction force pairs.

In this way, you can easily remember Newton’s 3rd law of motion.

“To every action, there is always an equal and opposite reaction”

Conclusion,

Whenever two objects come in contact with each other, they exert a force on each other.

So, here…

Consider, hand and spring are the two objects.

Therefore,

Whenever you press the spring, your hand comes in contact with the spring, right?

And both these objects apply forces on each other.

(As force always occurs in pairs)

So, according to Newton’s third law of motion,

As both the pair of forces act on two different bodies, these forces are known as Action-Reaction force pairs.

### #2 A Fish

Everyone has seen a fish swimming in the water, right?

But, many of you don’t know how Newton’s third law is applied here.

Do you know how Newton’s third law works when a fish moves forward in the water while swimming?

When fish is swimming in the water, the fish pushes the water backwards and the water pushes its body forward.

But, if I ask you to show the action-reaction force pair acting on the fish then how will you show?

Don’t worry, I’ll explain it to you in a simple way.

See this figure,

From the above figure, it is clearly seen that when fish moves forward, two forces are acting.

As the fish moves forward in the water, this is known as action force.

And at the same time,

Water also pushes the fish backwards, this is known as reaction force.

And you already know, force always occurs in pairs.

So, together these forces are known as action-reaction force pairs.

That’s how simple is the Newton’s third law of motion,

“To every action, there is always an equal and opposite reaction”

Conclusion,

Whenever two objects come in contact with each other, they exert a force on each other.

So, here…

Consider, fish and water are the two objects.

Therefore,

Whenever fish pushes the water backwards, fish comes in contact with the water, right?

And both “water” and “fish” apply forces on each other.

(As force always occurs in pairs)

So, according to Newton’s 3rd law of motion,

As these forces are acting on two different bodies, these forces are known as Action-Reaction force pairs.

### #3 Recoil of a Cannon

Have you seen a cannon in real life?

If you have not seen it yet, that is okay.

But do you know, what is the recoiling of a cannon?

When a cannon is fired,

Cannon exert a forward force on the cannonball and the cannon ball gets thrown away to a long distance.

But, have you noticed one thing?

When the cannon is fired, it moves slightly backwards.

This is known as the recoiling of a cannon.

(Which shows the presence of Newton’s third law of motion)

Here is the main part,

If the force exerted by the cannon on the cannon ball is same as the force exerted by the cannon ball on the cannon then,

Why doesn’t the cannon also recoils with the same acceleration as the cannon ball?

Has this question come into your mind?

Yes, the cannon recoils.

But, the cannon doesn’t recoil with the same acceleration as a cannonball, right?

Why?

The answer is simple for you. (If you have already referred to Newton’s second law of motion)

Newton has mentioned in his second law that the acceleration of any object inversely depends upon its mass.

Which means,

If the object has more mass, it’s acceleration will be less.

In this example, the mass of a cannon is very large compared to cannonball.

Hence,

The acceleration of a cannon will be very less than the cannon ball.

As acceleration inversely depends upon the mass. (According to Newton’s second law)

Now coming back to Newton’s third law,

The action-reaction force pairs acting on the cannon can be shown as follows…

As you can see,

The cannon exerts a forward force on the cannon ball. (This is action force)

And at the same time,

Cannon ball also exerts an equal and opposite force on the cannon. (This is reaction force)

So, together these pairs are known as action-reaction force pairs.

(As force always occurs in pairs)

That’s how simple is the Newton’s third law of motion,

“To every action, there is always an equal and opposite reaction”

### #4 The Boat

In the above example we have seen how cannon moves slightly backwards, when it is fired.

This is a similar example of a boat in which I’ll explain why the boat moves slightly backwards, when you jump from it.

Simply think, you are going on the boat for swimming.

Now, you have surely noticed that whenever you jump from the boat, the boat moves slightly backwards.

Why does this happen?

If you don’t know let me tell you,

Again this thing happens because of Newton’s third law.

As you can see in the below figure,

The boy jumps from the boat by pushing the boat backwards.

Now, If the force exerted by the boy on the boat is same as the force exerted by the boat on the boy then,

Why is the boat not moving with the same acceleration as the boy?

Has this question come into your mind?

Again,

The answer is simple for you, if you have already referred to Newton’s 2nd law of motion.

Newton has clearly mentioned in his second law that, the acceleration inversely depends upon the mass.

In this example,

As the mass of the boat is very large, its acceleration will be very less compared to the boy.

Now coming back to Newton’s third law,

The action-reaction force pairs acting on the boat can be shown as follows…

As you can see,

When the boy jumps from a boat, the boy exerts a force on the boat. (This is action force)

And at the same time,

The boat also exerts a force, back on the boy. (This is reaction force)

So, together these force pairs are known as action-reaction force pairs.

(As force always occurs in pairs)

That’s it.

All you need to remember is,

“To every action, there is always an equal and opposite reaction”

### #5 Flying Bird

Do you know, Newton’s third law is clearly seen while a bird is flying?

As you can see,

A bird is flying in the air by continuously pushing the air downwards with his wings.

Now according to Newton’s 3rd law of motion,

The action-reaction force pairs acting on the bird can be shown as follows…

As you can see,

A bird is continuously pushing the air downwards. (This is action force)

And at the same time,

Air also exerts an upward force on the wings of the bird. (This is reaction force)

So, together these pairs are known as action-reaction force pairs.

(As force always occurs in pairs)

These action-reaction force pairs make it possible for a bird to fly.

That’s it.

See, how simple to remember the statement of Newton’s third law.

“To every action, there is always an equal and opposite reaction”

### #6 Gunfire on a Skateboard

When gunfire is done standing on a skateboard, the skateboard will move slightly backwards.

Again, this thing happens because of Newton’s third law of motion.

When a gun is fired, the gun recoils and the skateboard moves slightly backwards. (As in case of cannon)

According to Newton’s third law, the action-reaction force pairs can be shown as follows…

As you can see,

When a gun is fired, the bullet moves in the forward direction. (This is action force)

And at the same time,

Due to recoiling of a gun, the man moves slightly backwards along with the skateboard. (This is reaction force)

As force always occurs in pairs, together these pairs are known as action-reaction force pairs.

That’s how you can easily remember Newton’s third law of motion,

“To every action, there is always an equal and opposite reaction”

If you want to read more examples of Newton’s third law of motion,

## Newton’s 3rd law – Misconception

Simply think,

What happens when two bodies come in contact with each other?

They exert a force on each other.

And force always occurs in pairs, right?

How, that I have already explained in the above examples.

Now, have you noticed one thing in the above examples,

All the action-reaction forces are equal in magnitudes and opposite in direction.

If you have read the first law of motion, you might be thinking this is similar to the definition of balanced force, right?

But, the question is,

Are the balanced forces and action-reaction forces both the same?

Or

The pair of forces in case of balanced force is the same as the pair of forces in case of Newton’s third law?

You might be thinking, the answer is YES, right?

If you are thinking so, then you are totally wrong.

That’s why, You need to first clear this misconception.

The most common misconception that people used to believe from a long time is,

“balanced forces” and “action-reaction forces” both are the same.

Are you believing the same? (Let me know in the comments)

(There is also one big Misconception discussed in Newton’s second law)

Let’s clear this Misconception !!

You have definitely read the example of feather and stone, in Newton’s second law, right?

Now, let’s consider that stone is simply lying on the ground.

I just want to ask you one simple question, is this stone is in balanced condition?

Obviously yes, right.

See this,

As you can see,

Gravity pulls the stone downwards and at the same time, ground also pushes the stone upwards.

Therefore, the stone is balanced by the vertical forces.

(As both these forces are of equal magnitudes and having opposite directions)

Have you noticed one thing in the above figure?

The upward and downward forces, both are acting only on the stone.

(i.e. forces are acting only on one object, which is a stone)

Always remember, balanced forces always act on the same object.

But,

If you are considering Newton’s third law of motion, then what do you think?

If I ask you to show the action and reaction forces acting on the stone then, how will you show?

Again If you are thinking,

The action and reaction force pairs and the balanced force pairs both should be the same.

Then, You are wrong.

See this,

When stone is placed on the ground,

The stone pushes the ground downwards, this is known as action force.

Similarly,

Ground also pushes the stone upwards, which is known as reaction force.

Therefore,

According to Newton’s third law of motion, these pair of forces are known as action and reaction forces.

In short,

Whenever two bodies come in contact with each other, they exert force on each other, right.

So, here… stone and ground are the two objects.

Whenever stone and ground come in contact with each other, both of them apply forces on each other.

The point is,

As both the forces are acting on two different objects, they are known as action and reaction forces.

In short, if you are considering newton’s third law of motion…

You can’t say, action and reaction forces and the balanced forces are the same.

(As they always act on two different objects)

### Conclusion

If you have still confusion in the balanced forces and the action and reaction forces,

Just remember these two main points:

• Balanced forces always act on the same object
• Action and Reaction forces always act on two different objects

Now, don’t you think it is easy to remember the statement of Newton’s 3rd law?

“To every action, there is always an equal and opposite reaction”

## Important points to remember

Newton’s 2nd law describes how much the body will accelerate further, when an external force is given to it.

Now according to Newton, this external force never occurs separately on the body.

So, Newton’s third law states that forces must always occur in pairs.

That’s why Newton has given the statement of third law in a very simple language.

“To every action, there is always an equal and opposite reaction”

In short, while understanding Newton’s 3rd law of motion, you need to remember these 3 points:

• Force always occurs in pairs
• There is no cause/effect relation
• Forces always act on two different bodies

Let’s understand these points one by one…

### 1. Force always occurs in pairs

Until now, you have definitely understood the whole statement of Newton’s 3rd law right?

And you already know, the terms “Action” and “Reaction” are nothing but the pairs of force.

Using different terms for the same concept, sometimes make you confused in order to remember the statement of Newton’s 3rd law.

Here is a simple way, by which you can easily remember the statement of Newton’s 3rd law of motion.

Consider two bodies A and B are walking towards each other.

Now, what happens when these two bodies come in contact with each other?

Obviously, they exert force on each other.

By this way, you can easily remember newton’s 3rd law of motion:

“Force on a body A by B is equal and opposite to the force on a body B by A

### 2. There is no cause/effect relation

Now you already know that force always occurs in pairs, right.

But,

Many of you have one misconception in your mind about the terms action and reaction that,

Action comes before Reaction.  (i.e. Action is the cause and Reaction is the effect)

Do you have the same misconception?

If you have the same misconception, then you are wrong.

There is no such thing mentioned in Newton’s third law of motion.

The force on body A by B and the force on body B by A act at the same instant.

Remember,

Anyone of these two forces can be called as action and the other as reaction.

### 3. Forces always act on two different bodies

After understanding the above two points, one Last Point you need to remember is,

Action and Reaction forces always act on two different bodies, not on the same body.

If you are considering bodies A and B then according to Newton’s 3rd law of motion,

FAB = – FBA

(Force on A by B) = – (Force on B by A)

As these forces act on two different bodies, they don’t cancel away.

For this reason, action-reaction forces and the balanced forces both are different.

## What are Interaction force pairs?

As you already know, force always comes in pairs.

Any one of these forces is action force and the other is reaction force.

Sometimes, these action-reaction force pairs are also known as Interaction force pairs.

Let’s understand the interaction force pairs by considering one simple example of a chair.

Have you read this example of a chair, which is discussed in the newton’s first law?

As the forces are balanced, the chair doesn’t break.

(Both the upward and downward force are equal in magnitudes and opposite in directions)

Now, if I ask you to show the interaction force pair, then how will you show?

See this,

When person sits on the chair,

The person pushes the chair, which is known as action force.

AND

The chair pushes the person, which is known as reaction force.

In short, these action and reaction forces are nothing but the interaction force pairs.

## Examples of Interaction force pairs

In the below examples I have used some examples which are already explained earlier in the first law.

From the below examples, you’ll know the exact difference between balanced forces and action-reaction forces.

Okay, so let’s move on to the examples…

### Dumbbells

When a dumbbell is placed on the table, it is in balanced condition, right?

See this,

As you can see, both the forces are of equal magnitudes and having opposite directions.

Now according to Newton’s 3rd law of motion, the interaction force pairs can be shown as follows:

As you can see,

In both the cases, the forces are of equal magnitudes and having opposite directions.

So, what’s the difference between the balanced force and the interaction force pairs (action and reaction forces)?

If you have noticed in the second case, forces are acting on two different objects.

Whereas in the first case, forces are acting only on the dumbbell.

Conclusion is,

If you are considering Newton’s third law of motion, then interaction forces always act on two different objects. (See the second case)

### The Book

When books are placed on the table, the books are said to be in a balanced condition.

As both the forces are of equal magnitudes and having opposite directions.

Now, according to Newton’s third law, the action and reaction forces (interaction force pairs) can be shown as follows:

Here also, the forces are of equal magnitudes and having opposite directions.

The only difference is in the second case, where the forces are acting on two different objects.

## Summary

It’s time to revise all the main points which we have seen till now in Newton’s third law of motion.

• What is Newton’s third law?

“To every action, there is always an equal and opposite reaction”

In the above statement, the terms “action” and “reaction” are nothing but a pair of forces.

In short, force always occurs in pairs.

• Misconception in Newton’s 3rd law

The big misconception that people used to believe from a long time is,

Both “balanced forces” and “action-reaction forces” are the same.

Are you believing the same?

If you have already referred the definition of balanced force in Newton’s 1st law, then you know…

In both the definitions, the forces are of equal magnitudes and having opposite directions.

So what’s the difference?

Always remember,

Balanced forces always act on the same object.

And, action and reaction forces always act on two different objects.

• Points to remember in newton’s 3rd law of motion

1. Force always occurs in pairs

2. There is no cause/effect relation

3. Forces always act on two different bodies

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Don’t you think, is easy to remember the statement of Newton’s third law of motion?

(Let me know by leaving a comment)