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Law of Inertia - Kinematics

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A Detailed Discussion of the Law of Inertia

Newton's first law of motion is also cited as the law of inertia.

This law states that a body continues to be in the state of rest or uniform motion along a straight line unless it is acted upon by an external force to change its state.

In our daily life, we find that a ball rolling on the ground stops after some time. This is because the frictional force of the ground is acting upon the ball to make changes in its state of motion.

The inertia of a body is measured by the mass of the body.

Heavier is the mass, greater is the force required hence, greater is its inertia and vice-versa.

Hence, Newton’s first law defines inertia, and it is justly called the law of inertia.


What is Inertia?

The inherent attribute of all the bodies, by virtue of which they oppose the change in the state of rest or of uniform motion along the straight line on their own, is called inertia.

Some real-life applications to describe the inertia are as follows:

  • Tables and chairs are kept inside the classroom and remain in a state of rest until someone moves them.

The reverse can also be true.

  • Suppose I am driving a car at a high speed. Suddenly, I encountered a big rock. I need to apply force to stop the car to bring it to a state of rest.


Inertia Examples

A few examples to describe the law of inertia in our everyday life are as follows:

  • When a bus starts suddenly, we fall backward. This is because our lower part of the body starts moving with the bus while the upper part tries to remain at rest.

  • Athletes run a certain distance before taking a long jump.

  • When a horse at full gallop stops suddenly, a man falls forward.

  • On shaking the mango tree, mango falls off the tree.

  • When a car rounds a curve suddenly, a driver is thrown outside.

Types of Inertia

Inertia is the resistance of a body to any change in its velocity.

It is of three types:

  1. The inertia of rest: Tendency of a body to remain in the state of rest.

  2. The inertia of direction: Tendency of a body to remain in a particular direction.

  3. The inertia of motion: Tendency of a body to remain in a state of uniform motion.

Inertia of Rest

A body tends to remain at rest until an external force is applied to make it move. Therefore, the inertia of rest is an inability of a body to move and remain in the state of rest.


Example of Inertia of Rest

When we beat the carpet, it comes in motion, and dust particles are in a state of rest. This is because the dust particles tend to remain at rest, and hence get separated.

Inertia of Direction

The body tends to remain in the same direction until an external force acts upon it to make a change in its direction of motion.


The Inertia of Direction Examples

A few Real-life Examples are discussed below:

  • A bike tends to move in a straight line unless we turn the handle on the bike.

  • You use umbrellas to prevent yourself from getting wet. The direction of the raindrops is vertically downwards.

  • They cannot change their direction to make you wet.

  • When you spin the one end of a string tied to the stone. Suddenly the string breaks and stoneflies off along the tangent to the circle. This is because the pull in the string was forcing the stone to make a circular motion.

As soon as the string breaks, the pull vanishes, and the stoneflies off tangentially. 


Inertia of Motion

The body continues to move in a uniform motion and the property under which it opposes the change in its present state is called the inertia of motion.


Examples of Inertia of Motion

Few real-life examples to describe inertia of motion are as follows:

  • When a train stops suddenly, we fall forward. By inertia of motion, the upper part of the body is in contact with the seat and starts moving with the train, while the lower part tries to remain at rest.

  • A person jumps out from a train and falls forward. This is because his feet are in contact with the ground and remain at rest while the remaining body continues to move because of inertia of motion.

Do you know?

Before Newton explained the three laws of motion, the great Greek thinker Aristotle put forward the concept that an external force is required to keep a body in motion. But he failed to realize that an opposite frictional force acts on the body to counter the external force so that the net force on the body is zero.

Sir Isaac Newton is regarded as the father of classical physics, owing to his laws of motion. The Law of Inertia is the name given to the first of these laws. The most significant and well-known is the law of inertia. Let's take a closer look at the first law of inertia in this essay.

Let's look at the definition of inertia before we go into the law of inertia. Unless acted upon by an external force, the matter has the property of inertia, which permits it to remain at rest or in uniform motion along a straight line.

What is the Inertia Law?

Newton's first law of motion, usually known as the law of inertia, asserts that unless an external force acts on an object, it will remain in either a condition of rest or motion. We have all heard of the Aristotle fallacy, which states that a body must always be propelled by an external force. When the concept of inertia was introduced, this was shown to be incorrect. Galileo discovered the notion of inertia with the following two tests.

Galileo and the Inertia Concept

The concept of inertia was created by Galileo, a leading scientist in the seventeenth century. According to Galileo, moving objects eventually come to a standstill due to a force known as friction. Galileo found that a ball would roll down one plane and up the opposite plane to roughly the same height in tests utilising a pair of inclined planes facing each other. The ball would roll up the opposite plane even closer to the original height if smoother planes were utilised. Any difference between the initial and final heights, Galileo reasoned, was attributable to friction. Galileo claimed that if friction were fully removed, the ball would soar to the exact same height.

Examples of the Law of Inertia

  • The lift began abruptly.

  • When a stationary bus begins to move, it has a tendency to move backwards.

  • When the lift suddenly operates, it causes a jerk.

  • When an abrupt break is applied, you can go forward.



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Last updated date: 09th Jun 2023
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FAQs on Law of Inertia - Kinematics

1.  What is the Importance of Inertia?

Inertia has a real-life application in our day-to-day work. Let’s discuss them:

To understand the reluctance of any physical object to make a change in its state. 

For example, in mission Mars, We just needed fuel to escape the rocket from the gravitational pull, and retard it when it reached Mars.

Inertia played a big role here, it carried the rocket for about 54.6 million Km in space from earth to Mars without any fuel.

2. Is Inertia a Force?

Yes, inertia is a force that keeps a stationary object at rest and moving objects in the same direction and at a constant speed. In an ideal case, an object won’t have inertia in a gravity-free environment. Hence, inertia is the natural tendency of a physical object to resist motion and eventually brings the body to a state of rest. Download the free pdf from Vedantu for the Law of Inertia - Kinematics to have a deeper understanding of the topic.

3. What is the formula of the law of inertia?

Inertia is the tendency of mass and matter to resist the change in motion. This can be well demonstrated by the 2nd Newton’s Law of Motion. Hence the formula is Net force = Mass Acceleration, or F = MA. Vedantu offers application-based problem equations in the Law of Inertia - Kinematics along with a basic understanding of the laws and concepts. Download the free PDF and you will be able to know the law of inertia properly. 

4. Is Kinematics related to Newton's law of motion?

Basic kinematics is defined as the motion of objects. Kinematics is studying the motion of the object on a broader basis. Basic kinematic problems are approached using Isaac Newton's laws of motion. Hence Newton's law of motion comprises kinematics as one of its applications. It can be well studied with the help of online classes conducted by Vedantu. Register now and join the classes for expert help.

5. What is Inertia of a Body?

Inertia is the natural tendency of a body to resist any change in its state. For example, while sleeping, you are in a free state. If someone tries to wake you up. You continue to sleep and try to resist that person from waking you up.

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