What is Newton’s Law of Inertia? [Practical Explanation]

When I have read the statement of newton’s first law of motion for the first time, 

I was wondering, what should be the statement for the Newton’s law of inertia?

Later on, I came to know that both these laws have the same statements.

I mean,

Both, Newton’s first law of motion and the Newton’s law of inertia are the same.

(Only their names are different)

See this, 

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.

Now the question is, 

Why is Newton’s first law of motion also known as the Law of inertia?

Let’s understand with some practical examples.

Types of Inertia

Before moving onto the examples of Newton’s first law of inertia, you should know about the basics of inertia.

Take a look, 

There are three different types of Inertia:

  1. Inertia of rest
  1. Inertia of motion
  1. Inertia of direction 

Let’s understand this practically. (One by One)

1. Inertia of rest

According to newton’s law of inertia,

Any object which is at rest, remains at rest unless an unbalanced force acts on it.

This is known as Inertia of rest.

For example, 

This bottle will not move on its own, unless you apply some external force on it. 

(Whether you push it or pull it)

2. Inertia of motion

According to newton’s law of inertia, 

Any object which is in motion, remains in motion unless an unbalanced force acts on it.

This is known as Inertia of motion.

For example, 

You have definitely experienced this, right?

That’s why you must wear the seat belts while traveling in the car.

3. Inertia of direction 

According to newton’s law of inertia, 

Any object which is in motion will continue its motion, with the same speed and in the same direction. 

(If there is no friction along the level surface)

This is known as Inertia of direction.

For example, 

This ball keeps on moving with the same speed and in the same direction, unless an unbalanced force acts on it.

(If there is no friction along the level surface)

Newton’s law of inertia Examples 

If you have properly understood the 3 different types of inertia mentioned above,

You’ll get an exact idea on why Newton’s first law of motion is also known as the Law of inertia.

Let’s move onto Newton’s law of inertia examples…

The Ball 

Do you know how Newton’s law of inertia applies to a football?

It’s simple.

Look: According to Newton’s first law of inertia

Objects will not change their behaviour unless an unbalanced force acts on them, right.

See this, 

This football will not start moving on its own, unless someone hits it or kicks it.

Football remains in its state of motion. (Football at rest remains at rest)

Even after someone kicks the football, the football won’t stop on its own, unless an unbalanced force acts on it.

This football will remain in its state of motion, unless the gravity force acts on it.

(Football keep on moving forward in the air, unless an unbalanced force acts on it)

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

(Objects keep on doing what they are doing, unless an unbalanced force acts on them)

Egg drop – Experiment

To perform this simple experiment all you need is, 

  • One egg
  • Empty toilet paper roll
  • Glass with water and
  • One dish

Make a set up using the above mentioned things as follows: 

Now, what happens if you hit the dish with your hand?

The egg falls straight into the glass and the dish gets thrown away, right.

(Only works, if you hit the dish forcefully)

Here one question arises in a mind that, 

Why not the egg also get thrown away, along with the dish?

The reason is simple.

As per the newton’s first law of motion

Objects will not change their behaviour unless an unbalanced force acts on them.

Therefore, 

When you hit the dish with your hand, the only force acting on the egg is the gravity force.

This gravity force pulls the egg straight down.

In other words, the egg has inertia.

Egg will not change its behaviour unless an unbalanced force acts on it.

(Objects tend to do what they are doing, unless an external force acts on them)

This thing indicates the presence of Newton’s law of inertia.

Apple tree

Have you ever wondered why the apple falls down, when you shake the apple tree?

It’s because of inertia.

As the branches of the apple tree have inertia, it will not change its behavior unless an unbalanced force acts on it.

In other words,

When you shake the tree, an unbalanced force acts on the branches of the tree as well as on the apple.

Due to this, the apple falls down on the ground. (Apple will not fall on its own)

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

(An object at rest remains at rest and object in motion remains in motion unless an unbalanced force acts on it)

Check out more examples here: Real life examples of Newton’s law of inertia.

Galileo’s concept of Inertia

In the 17th century, a scientist named Galileo formulated the concept of Inertia.

Galileo studied the motion of objects on an inclined plane.

Here are the some observations that Galileo observed:

  1. If objects are allowed to move down, they accelerate downwards on their own.
  1. If objects are allowed to move up, they don’t accelerate upwards on their own.
  1. If objects are allowed to move on a horizontal plane, they will accelerate forward.

Galileo concluded from these observations that, 

Objects which are moving on a frictionless horizontal plane, will neither have acceleration nor have retardation.

In other words, Objects should move with the constant velocity.

Galileo’s Experiment 

Galileo’s second experiment which involves a double inclined plane also made the same conclusion as above.

In this experiment,

A ball (initially at rest) when released from an initial point climbs up the final point of the curve.

Let’s take a look.

Case #1

If the planes are smooth, the final height of the ball is the same as the initial height.

(May be less, but can never be greater than the initial height)

See this,

In the absence of friction, 

The final height of the ball is the same as the initial height.

Case #2

If the slope of the second plane is slightly decreased and the same experiment is repeated.

The ball will still reach the same height.

See this, 

As you can see, the ball reaches the same height.

But on doing so, the ball travels up to a longer distance.

Case #3

In this case, 

The slope of the final plane is zero.

As you can see, 

Here the ball travels to an infinite distance, when the plane is horizontal. (i.e. the slope of the second plane is zero)

In other words, the motion of the ball never stops.

Yes, the ball stops because of friction.

But if there is no friction, the ball will continue its motion with the same speed and in the same direction.

Conclusion

Galileo concluded from above experiments that, 

Object will continue its motion, if no external force is given to it.

This property of an object is known as Inertia.

(Which means resistance to change)

Objects do not change their state of rest or uniform motion, unless an external force compels it to change that state.

In other words,

External force only helps in accelerating the object forward.

(Object will not change its behaviour, if no external force is given to it)

In the 17th century, 

Isaac Newton built on Galileo’s simple but revolutionary ideas and laid down the foundation of classical mechanics.

Newton has given the three laws of motion that go by his name.

Newton has given these three laws of motion:

Galileo’s law of inertia was his starting point, which he formulated as Newton’s first law of motion.

(Which is also known as the Law of inertia)

State of motion

As seen above, 

Inertia is the tendency of an object to resist the change in its state of motion.

Now the question is, 

What do you understand by this term, State of motion?

It’s simple.

“The state of motion of any object can be defined by its velocity

Let me explain.

If an object is at rest its velocity is 0 m/s right, which remains in its state of motion.

(Unless an unbalanced force acts on it)

See this, 

Remember this example, which is discussed in Newton’s first law of motion?

Here the ball is moving with the velocity, say… xyz m/s, which will keep on moving further with the same velocity in the same direction.

(If there is no friction or some kind of air drag acting along the level surface)

In other words, 

The ball remains in its State of motion. (Whether it is in rest or in motion)

Objects don’t change their State of motion, If no external force acts on them.

That’s why. As per the Newton’s law of inertia,

In order to change the state of motion of any object, an unbalanced force must be acted upon it.

(An object at rest remains at rest and object in motion remains in motion unless an unbalanced force acts on it)

Let’s understand this using practical examples.

The Book

Imagine,

While you are studying a book of physics, you’ll get tired and you have placed the book somewhere.

Now the question is, 

Will this book move on its own?

Obviously no, right.

See this,

As the book has inertia, the book will remain in its state of motion.

Unless an unbalanced force acts on the book, the book won’t change its behaviour.

In other words, the book remains in its State of motion.

An Eagle

Have you seen an eagle flying in the air?

It looks like an eagle is flying with the uniform velocity, right?

The point is, 

Consider if there’s no friction or air drag acting on the eagle.

In other words, 

Eagle remains in its State of motion.

(Eagle keep on flying with the uniform velocity, in the same direction)

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Don’t you think it is easy to remember the statement of Newton’s law of Inertia?

(Let me know by leaving a comment)

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8 thoughts on “What is Newton’s Law of Inertia? [Practical Explanation]”

  1. I love the simple way you have explained this, I am deeply impressed and you did such a great job with it. Thanks mate…extremely useful x

    Reply

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