In the 17^{th} century, **Issac Newton** gave the 3 laws of motion, which is further known to us as Newton’s laws of motion.

Newton has given these three laws of motion:

All these 3** **Newton’s laws of motion together laid the foundation of classical mechanics.

**Newton’s first law of motion states that:**

*“An object at rest remains at rest and object in motion remains in motion unless an **unbalanced force** acts on it”*

Newton’s first law is also known as the Newton’s first law of inertia.

**Newton’s second law of motion states that:**

*“The **acceleration** is directly proportional to **net force** applied and inversely proportional to **mass** of the object”*

**Newton’s third law of motion states that:**

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

If you are confused by reading the above statements, don’t worry.

I’ll have explained all the 3 laws of motion – step by step.

(With examples, equations and many more)

Let’s get started with Newton’s 1^{st} law of motion.

## Newton’s first law of motion

**Will this ball move?**

**Will this ball stop?**

In both cases, external force is needed to be given on the ball.

(Whether to bring the ball in motion **or** to stop its motion)

In short, **External force** is the only reason which puts a stationary body into motion and stops a moving body.

Now the question is, what happens if a body is moving with the uniform speed?

Still an external force is required to keep its motion continue?

What do you think?

In order to keep this skateboard moving, **does any external force is required?**

It’s not easy to answer this question, right.

Issac Newton has answered this question in the 17^{th} century, which is later known as the newton’s first law of motion.

**Newton’s first law of motion states that:**

*“An object at rest remains at rest and object in motion remains in motion unless an **unbalanced force** acts on it”*

Newton’s first law of motion is also known as the **law of inertia.**

### Examples of Newton’s first law

After reading the statement of Newton’s first law, you might get confused, right?

Don’t worry.

I’ll make you understand the statement of Newton’s 1^{st} law with some real life practical examples.

So without wasting time, let’s move on to the examples.

**Example #1**

Newton’s first law of motion can be clearly understood by the example of a car.

Simply think,

When you are going somewhere in the car, the car doesn’t start on its own right.

Unless you start the car, the car will remain in rest condition only.

See this,

As you can see,

The car will not start on its own, unless the boy starts it with the key.

Okay, the car is in motion now.

Now the question is, will this car continue moving forever?

**Obviously no,** right.

The car will stop when you apply brakes, right.

When the brakes are applied to the car, it is said that an **unbalanced force** acts on the car.

In short,

The car will not change its behaviour, unless an **unbalanced force** acts on it, which shows the presence of Newton’s 1^{st} law of motion.

(Whether the car is moving or simply in rest position)

**Example #2**

Do you know how Newton’s first law of motion is applied in the game of pool?

Don’t worry. I’ll make this simple.

See this,

As you can see, the ball comes into motion when one girl hits the ball with the pool stick.

What does it mean? (It’s simple)

It means that external force is applied to the ball with the help of a stick, which allows the ball to come in motion.

**Here is the interesting part,**

You know what, the ball which is in motion will remain in motion. (If **no external force** acts on it)

In short, **external force** is the only reason which stops a moving ball and puts a stationary ball into motion.

(This external force behaves as an **unbalanced force)**

In short,

The ball will not change its behaviour, unless an **unbalanced force** acts on it.

(Whether the ball is moving or simply lying on the table)

This thing shows the presence of Newton’s first law of inertia.

**Example #3**

This is a real life example of Newton’s first law of motion.

(You have definitely experienced this, while riding a bicycle)

Simply think, you are going somewhere on your bicycle.

Will your bicycle start moving on its own?** (Obviously no)**

See this,

Here, both the bicycles are initially at rest. (along with the boys)

In short, the bicycle will not move on its own unless an** external force** is applied to it.

The bicycle comes in motion, only when both the boys apply some external force on it.

Also after coming into motion, the bicycle will remain in motion. (If **no external force** applied to it)

When the boy strikes with the obstacle, the bicycle stops.

Even after the accident, the boy will continue moving forward.

(If there is **no such unbalanced force** acting on the boy to stop its motion)

Therefore,

According to Newton’s 1^{st} law of motion, the bicycle will not change its behaviour unless an **unbalanced force** acts on it.

### What is balanced force?

*“Those forces which are having **same magnitudes** and **opposite directions** are known as **balanced forces**“*

Let’s make this simple.

The concept of balanced forces can be easily understood while playing the game of tug of war.

See this,

Here, both the teams are applying forces on each other by pulling the rope on their side.

As these forces are of equal magnitudes and having opposite directions, they are known as **balanced forces.**

That’s why it is mentioned in the statement of the newton’s first law,

The object will only accelerate if an **unbalanced force** acts on it.

(If the forces are balanced, the object will not change its behaviour)

*“An object at rest remains at rest and object in motion remains in motion unless an **unbalanced force** acts on it”*

### What is unbalanced force?

After reading the above statement of the newton’s first law,

A question arises in mind that… What is an unbalanced force?

The definition of unbalanced force can be given as,

*“The forces that **cause a change in motion** of a body are known as **unbalanced forces**“*

Let’s understand this definition by considering one example.

As both the forces are balanced, the box is not moving from its original position.

This box will move easily when these boys push the box from the opposite sides.

This thing shows that the forces are **unbalanced.**

In short, those forces that **cause a change in motion** of a body are known as **unbalanced forces**.

### Conclusion

Don’t worry, if you still have some doubts left in your mind.

Always remember,

**If the forces are balanced,**the body will not change its behaviour**If the forces are unbalanced,**the body will change its behaviour

### What if the forces are balanced?

According to Newton’s first law of motion, the object will change its behaviour only when an **unbalanced force** acts on it.

Now the question is, what happens if the forces are balanced?

From the below flowchart,

You’ll get an exact idea on why objects don’t change their behaviour, when they are in **balanced condition.**

If the object is at rest, it will remain in rest.

And

If the object is in uniform motion, it will continue to stay in uniform motion. (With the same speed and in the same direction)

### What is inertia?

In simple terms, Inertia is** Inactiveness.**

**Or **you can remember Inertia as, **tendency to do nothing**.

Let’s** **take one practical example, so that you’ll get an exact idea.

You have definitely heard about the very famous experiment** **of** glass and coin, **right?

Don’t worry, if you haven’t heard yet.

**You can perform it now,** just by making a setup as mentioned in the above figure.

Now when you hit the card forcefully, the coin falls straight into the glass.

See this,

This thing indicates that the **coin has inertia.**

As the coin has inertia, the coin will remain in its original position and the card gets thrown away.

That’s why, Newton’s first law of motion is also known as** **the **law of inertia.**

In fact,

Newton’s law of inertia can also be experienced while wearing a seat belt.

If you have noticed, seat belts won’t let you move forward when the brakes are applied in the car.

In this case, as the boy has inertia, the boy will not change its behaviour.

Seat belt stops the boy by exerting an **unbalanced force** on the boy.

If there were no seat belts, the boy will continue its motion in the forward direction.

## Newton’s second law of motion

**Why is it easy to move this stone?**

**Why is it difficult to move this stone?**

In both cases, **external force** is required to move the stone, right.

(As per the first law of motion, the object will only accelerate if an unbalanced force acts on it)

Now the question is, why is it easy to move the stone in the 1^{st} case and why is it difficult to move the stone in the 2^{nd} case?

The answer is simple.

As the stone has **less** **mass,** it’s easy to move the stone and as the stone has **more mass,** it’s difficult to move the stone.

In short, the **acceleration** of stone depends upon both **force** as well as **mass.**

Therefore, Newton’s second law of motion is all about understanding these 3 terms: acceleration, force and mass.

**Newton’s second law of motion states that:**

*“The **acceleration** is directly proportional to **net force** applied and inversely proportional to **mass** of the object”*

### Examples of Newton’s second law

Well such statements are quite complicated to understand.

Don’t worry, I’ll make this simple.

Check out the below examples. **(You’ll get the exact idea)**

**Example #1**

Why is it difficult for one single horse to pull the cart?

It’s simple. (As the cart has more mass)

In short, acceleration of cart is **inversely proportional** to its mass.

So, how will this cart move?

When two to three horses helps in pulling the cart, the cart accelerates easily.

The reason is simple.

The net force applied on the cart by these horses is **very large **compared to one single horse.

(So the cart accelerates further easily)

In short, acceleration of cart is **directly proportional** to net force applied on it.

Therefore,

According to newton’s second law of motion,

*“The **acceleration** is directly proportional to **net force** applied and inversely proportional to **mass** of the object”*

**Example #2**

Newton’s second law of motion can be clearly understood when you hit the ball by the bat.

See this,

Look,

According to Newton’s 2^{nd} law of motion,

Acceleration of an object is **inversely proportional** to its mass, right.

Therefore,

The player has to apply more force to hit the football. (As the mass of the football is more)

Now, with less amount of force, the cricket ball accelerates to a longer distance.

As the mass of the cricket ball is less, the player has to apply less amount of force.

Therefore, acceleration of a cricket ball **directly depends** upon the net force applied to it.

(Which indicates the presence of Newton’s 2^{nd} law of motion)

### Newton’s 2^{nd} law – Misconception

After reading the statement of Newton’s 2^{nd} law of motion, anyone can speak this law word to word.

Now,

Newton’s 1^{st} law of motion says that,

In order to change the behaviour of an object some **external force **must act on it.

But the question is,

*In order to keep the object moving,* external force should be given to it continuously?

What do you think?

If you are thinking so, you have a Misconception in Newton’s second law.

**Even today,** people are believing this misconception.

Okay, let’s make this simple.

You have definitely played with the toy car, right?

Now when you release this car, how long will it take further?

Obviously, after some time it will stop. (Because of friction)

But, **if the friction is somehow neglected** then?

Still an external force is needed to be given on a car to keep its motion continue?

The answer is NO.

**Always remember,**

External forces don’t produce the motion of an object.

(It only helps in accelerating the object forward)

Remember the definition of Newton’s 1^{st} law,

“Object will not change its behaviour, if *no external force** is given to it*“

### Newton’s second law in terms of Momentum

In terms of Momentum, Newton’s 2^{nd} law of motion can be stated as:

*“The **net force** applied on the body is equal to its **rate of change of momentum** and takes place always in the direction of force applied”*

Let’s understand this statement step by step.

First of all, what is momentum?

It’s simple.

*“**Momentum** is that quantity by which how much **amount of motion present** in the body can be known”*

Symbol of momentum: **p**

Formula of momentum: **p = mv**

Unit of momentum: **kg m/s ^{2}**

You can remember the definition of **Momentum** as the product of **mass** and **velocity.**

For example,

As the momentum of the cricket ball is less, it causes **less injury.**

Whereas,

Football gives **more injury** compared to a cricket ball. (Because of the high momentum)

In short,

With the increase in mass of an object, its momentum also increases.

Now the question is**, **

How does a small bullet have **extremely high momentum?**

As it has a small mass, its momentum should be less right?

To understand these questions, you need to first understand the importance of velocity in Newton’s 2^{nd} law of motion.

So let’s move on to our next topic.

### Importance of velocity in Newton’s second law?

When a bullet is fired from a gun, it can cause a **serious injury,** right. (Because of its extremely high momentum)

So, why does the bullet which has a very small mass have extremely high momentum?

Okay, I’ll make this simple.

See this,

Only because of the high velocity, a bullet travels with extremely high momentum.

So,

Momentum is also **directly proportional** to velocity. (Along with the mass)

As seen above, **Acceleration** of any object depends upon both **mass** as well as **force.**

Similarly,

**Momentum** also depends upon both **mass** as well as **velocity**.

That’s how you can remember the statement of Newton’s 2^{nd} law of motion in terms of momentum.

*“The **net force** applied on the body is equal to its **rate of change of momentum **and takes place always in the direction of force applied”*

Let,

F_{net} = net force applied on the body

m (v – u) / t = rate of change in momentum

mu = initial momentum

mv = final momentum

u = initial velocity of the object

v = final velocity of the object

t = time

So, we get the following equation for newton’s 2^{nd} law in terms of momentum :

F_{net} = m (v – u) / t

F_{net} = (mv – mu) / t

### Newton’s second law equations

You need to remember these 3 equations in Newton’s second law of motion.

**Equation 1:**[F_{net}= ma]

**Equation 2:**[p= mv]

**Equation 3:**[F_{net}= m(v-u) / t]

Below are some solved numericals based on the above equations of Newton’s 2^{nd} law of motion.

(You can practice these numericals, Right Now)

**Numerical #1**

A ball having a mass of 4 kg is accelerating forward at the rate of 6 m/s^{2}. Calculate the net force applied on the ball.

**Solution:****Given data:**

Mass of the ball, m = 4 kg

Acceleration of the ball, a = 6 m/s^{2}

The net force applied on the ball, F_{net} =?**According to Newton’s second law formula,**

F_{net} = ma

F_{net} = 4 × 6

F_{net} = 24 N

Therefore, a net force of **24 N** is applied on the ball.

**Numerical #2**

When a net force of 100 N is applied to the wheel, it accelerates further at the rate of 25 m/s^{2}. Calculate the mass of the wheel.

**Solution:****Given data:**

The net force applied on the wheel, F_{net} = 100 N

Acceleration of the wheel, a = 25 m/s^{2}

Mass of the wheel, m =?**According to Newton’s second law formula,**

F_{net} = ma

m = F_{net }/ a

m = 100 / 25

m = 4 kg

Therefore, the mass of the wheel is **4 kg.**

**Numerical #3**

A bullet which is having some amount of momentum is moving forward with the velocity of 200 m/s. If the mass of the bullet is 60 grams, calculate its momentum.

**Solution:****Given data:**

Velocity of the bullet, v = 200 m/s

Mass of the bullet, m = 60 grams

Now, 1 kg = 1000 grams (So, 60 grams = 0.06 kg)

The momentum of a bullet, p =?**According to the formula of momentum,**

Momentum (p) = mass (m) × velocity (v)

p = m × v

p = 0.06 × 200

p = 12 kg m/s

Therefore, the momentum of a bullet is **12 kg m/s.**

**Numerical #4**

A scooter which is having a momentum of 120 kg m/s is moving with the velocity of 4 m/s. What will be the mass of the scooter?

**Solution:****Given data:**

Momentum of the scooter, p = 120 kg m/s

Velocity of the scooter, v = 4 m/s

Mass of the scooter, m =?**According to the formula of momentum,**

Momentum (p) = mass (m) × velocity (v)

p = m × v

m = p / v

m = 120 / 4

m = 30 kg

Therefore, the mass of the scooter is **30 kg.**

**Numerical #5**

A 600 kg truck is initially at rest. The truck reaches the velocity of 20 m/s in 15 seconds. How much net force is acting on the truck?

**Solution:****Given data:**

Mass of the truck, m = 600 kg

Initial velocity of the truck, u = 0 m/s

After time, t = 15 seconds

Final velocity of the truck, v = 20 m/s

The net force acting on the truck, F_{net} =?**According to Newton’s second law formula,**

F_{net} = m (v – u) / t

F_{net} = 600 (20 – 0) / 15

F_{net} = 40 × 20

F_{net} = 800 N

Therefore, a net force of **800 N** is acting on the truck.

**Numerical #6**

A bicycle that has a mass of 3 kg is moving forward with the initial velocity of 25 m/s. When the brakes are applied, an **opposing force** of 12 N acts on the bicycle. How much time will the bicycle take to stop?

**Solution:****Given data:**

Mass of the bicycle, m = 3 kg

Initial velocity of the bicycle, u = 25 m/s

When the brakes are applied, an **opposing force** of 12 N is applied on the bicycle.

Force applied on the ball, F_{net} = **– 12 N**

After what time, t =?

Final velocity of the bicycle becomes zero, v = 0 m/s**According to Newton’s second law equation,**

F_{net} = m (v – u) / t

(- 12) = 3 (0 – 25) / t

(- 12 × t) = 3 × (- 25)

(- 12 × t) = (- 75)

12 t = 75

t = 75 / 12

t = 6.25 seconds

Therefore, after **6.25 seconds** the bicycle will stop.

### What are your views?

**Test #1**

This car has a mass of 750 kg.

When the car accelerates further at a rate of ‘a’ m/s^{2}, a force of 3000 N acts on it.

At how much rate will the car accelerate forward?

**A) **2 m/s^{2 }

**B) **8 m/s^{2}

**C) **4 m/s^{2}

**D) **5 m/s^{2}

**Test #2**

The boy exerts a net force of ‘F’ N on the box such that the box will accelerate further at a rate of 5 m/s^{2}.

If the mass of the box is 8 kg, how much force is exerted on the box?

**A)** 60 N

**B)** 50 N

**C)** 30 N

**D)** 40 N

**Test #3**

This stone falling from some height gains the momentum of 800 kg m/s.

If the mass of the stone is 32 kg, with what velocity the stone will collide to the ground.

**A) **75 m/s

**B) **25 m/s

**C) **50 m/s

**D) **80 m/s

**Test #4**

A small bird gains the momentum of 35 kg m/s while flying in the air.

If the bird is flying with the velocity of 7 m/s, what do you think about the mass of a bird?

**A) **6 kg

**B) **4 kg

**C) **8 kg

**D) **5 kg

**Test #5**

A roasted chicken dish which has a mass of 3 kg is simply lying on the floor.

The dish moves forward with the velocity of ‘v’ m/s, when a force of 6 N is applied to it.

After 4 seconds, what will be the velocity of the dish?

**A) **9 m/s

**B) **8 m/s

**C) **5 m/s

**D) **3 m/s** **

**Test #6**

A football which is having a mass of 350 grams is moving forward with the initial velocity of 80 m/s.

When one player hits this ball, an **opposing force** of 4 N acts on it.

If the velocity of the ball slowly decreases and reaches to 25 m/s, after how much time the ball will stop?

**A) **5.7 seconds

**B)** 3.9 seconds

**C) **9.8 seconds** **

**D) **4.8 seconds

### What is Acceleration due to gravity?

You already know the value of acceleration, right?

Obviously yes, it’s **9.81 m/s**** ^{2}**.

Now the question is, how you’ll define acceleration due to gravity?

It’s simple.

*“The acceleration gained by an object because of the **gravitational force** is known as acceleration due to gravity”*

If the object is **free falling,** its acceleration will be** 9.8 m/s**** ^{2}**.

This value of acceleration is known **acceleration due to gravity** and it is denoted by the symbol **‘g’**.

### What is Gravitational field strength?

As mentioned above, the acceleration of any **free falling** object is **9.8 m/s**** ^{2}**.

Now I’ll explain the exact meaning of gravitational field strength.

Let’s directly move on to the example.

As you can see, hippopotamus experiences a larger **gravity force** as compared to a small dog.

And obviously, the acceleration of hippopotamus will be more as compared to small dog.

Because,

According to the statement of Newton’s second law,

Acceleration of any object directly depends upon the net force applied on it.

In short,

Gravitational field strength is defined as the ratio of *gravitational force per unit mass.*

The formula for the gravitational field strength is **F/m** and it is denoted by the symbol **‘g’**.

The value of gravitational field strength varies based upon the different locations.

**On the Earth’s surface, its value is 9.8 N/kg.**

(It means that every object on earth experiences 9.8 N of force upon every 1 kg of mass within the object)

### What is free fall?

*“Free fall** is that type of motion of the object in which object falls **only under the effect of gravity force”*

Any object which is free falling, experiences the only **gravity force.**

In case of free fall, all the objects fall at the same rate.

Let’s take one practical example.

Simply think,

What happens when you drop a small feather and a large stone at the **same time** and from the **same height**?

Which of these two objects will reach the ground first?

The answer is simple.

Both, feather and the stone will reach the ground at the **same time.**

See this,

It’s not surprising for you, if you have read the definition of free fall properly. (Read Again)

**Always remember, **

If you are considering a free falling motion, all objects fall at the same rate. (Because of the absence of air resistance)

Now the question is, what if there is some kind of air resistance present?

### Free fall with air resistance

Before understanding the concept of free fall with air resistance, let’s first understand the basic definition of air resistance.

So, what is air resistance?

In simple words, air resistance means one kind of **air drag.**

Let’s understand with the above example of a feather and stone.

As per the above example, what do you think which one will reach the ground first? (Feather or a stone)

(If both the objects are **experiencing** **air resistance**)

**Nothing to surprise here.**

In this case,

When both these objects fall downwards, they start experiencing an upward air resistance, right.

In short,

In case of air resistance, massive objects fall faster than less massive objects.

(As the stone experiences **more air resistance,** it falls faster than the feather)

**Now the question is,**

Why does the stone fall faster than a feather?

As the air resistance should slow down its speed, right?

To understand these questions, let’s move on to the next topic which is **terminal velocity**…

### What is Terminal velocity?

Terminal velocity is the **final velocity** of the object.

Don’t worry, I’ll explain how.

In the above case,

As the stone falls downwards, the stone keeps on experiencing air resistance in the upward direction.

(Until the upward and downward forces gets balanced)

See this,

As you can see, the feather reaches the terminal velocity in a **very short interval of time.**

On the other hand, the stone never reaches the terminal velocity.

(As there is no such force which balances the downward force of gravity)

The stone falls faster than the feather because it** **never reaches the terminal velocity.

**(Stone needs to accelerate more in order to get balanced)**

## Newton’s third law of motion

**The horse kicks the man?**

Or

**The man kicked the horse?**

The answer is simple.

Both, man and horse are kicking each other, right.

See this,

**To every action,**

**there is always an equal and opposite reaction.**

This way you can easily remember the statement of the newton’s third law of motion.

**Newton’s third law of motion states that:**

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

### Newton’s third law examples

In the above statement of Newton’s third law, the terms **action** and **reaction** are nothing but the **force pairs.**

In short, force always occurs in pairs.

**Don’t worry, **I’ll make this simple.

Below are some real life examples of Newton’s third law of motion.

(You’ll definitely understand the complete statement of newton’s third law)

So let’s move on to the examples…

**Example #1**

Do you know how Newton’s third law of motion works when you jump off the boat?

As you can see, the boat moves slightly backwards. (When the boy jumps off the boat)

According to Newton’s third law,

Every action has an equal and opposite reaction.

See this,

When the boy jumps from a boat, it indicates the **action force.**

At the same time,

The boat also exerts an equal and opposite force on the boy, which indicates the **reaction force.**

Which shows the presence of Newton’s third law of motion.

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

**Example #2**

How does Newton’s third law work when you simply press the spring with your hand?

Have you felt something in your hand while pressing the spring?

See this,

When you press the spring, spring also gives an equal and opposite reaction force on the hand.

This happens because of Newton’s third law of motion.

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

**Example #3**

How Newtons’ third law of motion works when a cannon is fired?

It’s simple.

When the cannon is fired, cannonball also exerts an equal and opposite reaction force on the cannon.

See this,

Always remember, force always occurs in pairs.

When the cannon is fired, it is known as the **action force.**

At the same time, cannon ball also exerts an equal and opposite **reaction force** back on the cannon.

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

### Newton’s 3^{rd} law – Misconception

The biggest misconception in newton’s third law of motion is, **balanced forces** and **action & reaction forces** are the same.

I’ll make you understand how, don’t worry.

Tell me one thing,

A stone which is simply lying on the ground is in balanced condition or unbalanced condition?

**Obviously, **it is in balanced condition.

See this,

As both the upward and downward forces are equal in magnitudes and opposite in directions, the stone is in balanced condition.

But always remember,

If you are considering Newton’s third law of motion, then these two forces are not said to be balanced forces.

In case of Newton’s third law of motion, forces always act on **two different bodies.**

Whereas in case of balanced forces, the forces always act on the **same body.**

**Always remember,**

Balanced forces always act on the same object.

Whereas,

Action and Reaction forces always act on two different objects.

### Important points to remember

Here are the 3 main points that you should keep in your mind while referring to Newton’s third law of motion.

**Forces always occurs in pairs**

Let’s make it easy by considering two bodies **A** and **B**.

Consider these two bodies walking towards each other.

Now as they strike with each other, they exert forces on each other. (As forces always comes in pairs)

In short, you can remember the statement of newton’s third law of motion as follows:

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

**There is no cause/effect relation**

After understanding the above point,

A question arises in mind that… which of these forces comes first? (Action or Reaction)

There is no such thing mentioned in the statement of 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.*

(Anyone of these two forces is the **action** **force** and the other is the **reaction force)**

**Forces always act on two different bodies**

One last important point you should keep in your mind, while reading the statement of Newton’s third law of motion.

Action and Reaction forces always act on two **different bodies**.

According to newton’s third law of motion,

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

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

**F**_{AB}** = – F**_{BA}

Always remember,

These two forces never cancel away. (As they act on two different bodies)

### What are interaction force pairs?

According to Newton’s third law of motion, forces never occur separately on the body.

(Force always comes in pairs)

One is the **action force,** and the other is the **reaction force.**

Together, these pairs of forces are known as **Interaction force pairs.**

Now the question is, how these interaction force pairs act on different objects?

Don’t worry, Let’s take some real life examples of the interaction force pairs.

**Example #1**

If you have read the first law of motion, then you already know why this chair doesn’t break on its own.

(As I have already discussed this example of chair in the Newton’s fist law of motion)

Only because of the **balanced forces,** the chair doesn’t break.

(As both the upward and downward forces acting on the chair are having equal magnitudes and opposite directions)

Now according to newton’s 3^{rd} law of motion, Interaction force pairs can be shown as follows:

Always remember,

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

**Example #2**

Do you know how the interaction force pairs are shown? (Simply when a dumbbell is placed on the table)

See this,

The only difference between the balanced forces and interaction force pairs is,

Interaction force pairs always act on **two different bodies.**

**Example #3**

According to Newton’s 3^{rd} law of motion, every action has **always** an equal and opposite reaction.

Here, the interaction force pairs are shown when you simply put some books on the table.

According to Newton’s 3^{rd} law of motion, Interaction force pairs always act on two **different objects.**

## Summary – All you need to know (Fast)

So, the question is…

There are total how many newton’s laws are there?

There are 3 newton’s laws of motion:

**Newton’s 1**^{st}law of motion**Newton’s 2**^{nd}law of motion**Newton’s 3**^{rd}law of motion

**Let’s revise** all the main points which we have seen till now in all the three Newton’s laws of motion.

### Newton’s first law of motion

**What is Newton’s first law of motion?**

*“An object at rest remains at rest and object in motion remains in motion unless an **unbalanced force** acts on it”*

Newton’s first law of motion is also known as the **Law of inertia**.

**What is balanced force?**

*“Those forces which are having **same magnitudes** and **opposite directions** are known as **balanced forces**“*

**What is unbalanced force?**

*“The forces that **cause a change in motion** of a body are known as **unbalanced forces**“*

**Conclusion**

If the forces are** balanced,** the body will not change its behaviour.

If the forces are** unbalanced,** the body will change its behaviour.

**What is inertia?**

Inertia can be defined as,** tendency to do nothing**.

(In simple words, Inertia is **inactiveness**)

### Newton’s second law of motion

**What is Newton’s second law of motion?**

*“The **acceleration** is directly proportional to **net force** applied and inversely proportional to **mass** of the object”*

(**Acceleration** of any object depends upon both **force** as well as **mass**)

**Newton’s 2**^{nd}**Law – Misconception**

The biggest misconception in Newton’s second law of motion is,

In order to keep the object moving, **external force** should be given to it continuously.

**Even today,** people still believe this misconception.

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

As per the 1^{st} law of motion, an object will change its behaviour only if an unbalanced force acts on it. (Otherwise NOT)

**Always remember,**

External forces don’t produce the motion of an object.

(It only helps in accelerating the object forward)

*“Object will not change its behaviour, if **no external force** is given to it”*

**Newton’s second law in terms of Momentum**

In terms of Momentum,

Newton’s 2^{nd} law of motion can be stated as:

*“The **net force** applied on the body is equal to its **rate of change of momentum** and takes place always in the direction of force applied”*

**What is Momentum?**

*“**Momentum** is that quantity by which how much **amount of motion present** in the body can be known”*

Symbol of momentum: **p**

Formula of momentum: **p = mv**

Unit of momentum: **kg m/s ^{2}**

**Newton’s second law equations**

You have to remember these three equations in Newton’s second law of motion:

**Equation 1: **[F_{net} = ma]

**Equation 2:** [p= mv]

**Equation 3:** [F_{net} = m(v-u) / t]

By using these three equations, you can easily solve different problems in Newton’s second law.

**What is acceleration due to gravity?**

*“The acceleration gained by an object because of the **gravitational force** is known as acceleration due to gravity”*

**Acceleration due to gravity,** **g = 9.8 m/s**^{2}

**What is gravitational field strength?**

Gravitational field strength is defined as the ratio of** ***gravitational force per unit mass.*

The symbol for the gravitational field strength is **‘g’** and in case of free fall, its value is 9.8 m/s^{2}.

**(Within Earth’s surface, its value is 9.8 N/kg)**

**What is free fall?**

*“Free fall** is that type of motion of the object in which object falls **only under the effect of gravity force”*

Any **object which is free falling, **experiences the only **gravity force.**

**What is air resistance?**

In simple words, air resistance means one kind of **air drag.**

In case of air resistance, massive objects fall faster than less massive objects.

**What is terminal velocity?**

Terminal velocity is the **final velocity** of the object.

Any object which is falling downwards is said to have reached its terminal velocity, when the **air resistance force** (in the upward direction) and **gravity force** (in the downward direction) gets balanced.

After that,

Objects will continue to fall towards the ground with this terminal velocity.

### Newton’s third law of motion

**What is Newton’s third law of motion?**

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

(Remember, force always occurs in pairs)

**Newton’s third law – Misconception**

The biggest misconception in newton’s third law of motion is, balanced forces and action & reaction forces are the **same.**

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

**Always remember,**

Balanced forces always act on the same object.

Whereas,

Action and Reaction forces always act on two different objects.

**Points to remember in newton’s 3**^{rd}**law of motion**

While understanding the statement of newton’s third law of motion, you should remember these 3 main points:

**1.** Force always occurs in pairs

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

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

**........**Don’t you think, is easy to remember the statement of all the 3 Newton’s laws of motion?

(Let me know by leaving a **comment**)