Law of Inertia - Kinematics

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 in 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 sometime. 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 the inertia.

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

Tables and chairs are kept inside the classroom and remain in the 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 a force to stop the car to bring it to the state of rest.

 Inertia Examples

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

  • When a bus starts suddenly, we fall backwards. 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. Inertia of rest: Tendency of a body to remain in the state of rest.

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

  3. 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, 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 in the state of rest. This is because the dust particles tend to remain at rest, and hence got separated.

Inertia of Direction

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

Inertia of direction examples

Few Real-life Examples are Discussed Below:

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

  • You use umbrellas to prevent yourself from getting wet. The direction of the rain drops 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 stone flies 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 stone flies off tangentially. 

Inertia of Motion

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

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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.

FAQ (Frequently Asked Questions)

Q1: What is Inertia of a Body?

Ans: 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.  

Q2: What is the Importance of Inertia?

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

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

For example, in mission Mars, We just needed fuel to escape the rocket from 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. 

Q3: Is Inertia a Force?

Ans: Yes, inertia is a force which 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 the state of rest.