Difference Between Momentum and Inertia

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What is Inertia?

Newton’s first law

This law states that a body remains at rest or in the state of motion unless an effort or force is applied to make any change in its position.

For example, when we push a lawn roller, it starts dragging on the ground.

It means the force is applied to make changes in its position otherwise, it is in the rest position and would have remained at rest for an infinite period.

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Another example, a bob attached to the string when brought to either end from its mean position, and then set free, starts oscillating about its mean position. 

As it is kept in the air medium so, there will be an invisible air resistance acting on the pendulum. This will eventually bring the bob to rest. So, this air acts as an external force.

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Types of Inertia

According to Newton’s first law of the law of inertia, a body cannot make any change in its position, unless it is acted upon by an external force.

The inertia of a body is of three types:

  1. Inertia of rest

  2. Inertia of motion

  3. Inertia of direction

Inertia of Rest

The resistance offered by a body to change in its state of rest. This means the body remains at rest and cannot start moving on its own.

Inertia of motion

The resistance offered by the body to change its state of uniform motion. This means the body is in uniform motion and can neither be accelerated nor retarded on its own, eventually comes to rest.

Inertia of Direction

The resistance offered by a body to change in its direction of motion along the straight line.

This means the body continues to traverse a linear path until some external force changes its direction of motion.

Inertia Examples

1. Inertia of Rest

The real-life examples of inertia at rest are outlined hereunder:

  1. Place a coin on a smooth piece of cardboard covering the jug. Strike the cardboard piece with your finger suddenly. The cardboard slides and the coin falls into the jug. This happens on account of the inertia of rest.

2. Inertia of Motion

  1. An airplane travels some distance on the ground before it can take-off. This is because it has a fixed aircraft wing, which can produce lift only when there is a relative velocity between the airplane and the air. 

          So, moving a certain distance, the airplane gains velocity.

3. Inertia of Direction

  1. When a knife is sharpened by pressing against a grinding wheel, the sparks fly off along the tangent to the grinding wheel, on account of the inertia of direction.

What is Momentum?

The momentum of a body or an object is the quantity of motion contained in it.

The quantity of motion in a body can be created or destroyed by the application of force on it; however, its momentum remains conserved. Therefore, it is measured by the force required to stop the body in a given time.

The force required to stop a moving body depends upon the following:

a. Mass of the Body

When a ball and a big piece of stone are made to fall from the same height, and at the same time. 

We found that a much greater force is required to stop the stone as it possesses greater mass. That’s why the stone has greater momentum.

b. The Velocity of the Body.

A bullet thrown with hands can be stopped easier than the same fired from the bullet. This is because velocity is greater in the latter case, therefore, with a larger velocity of the body, greater will be its momentum.

Momentum Examples

1. A truck and the car traveling at the same speed suddenly encounter a big rock on their way. Here, the truck will slow down long before the car.

By the relation, p = mv, the truck has a larger mass, but lesser velocity, so it will stop long before the car stops because the car has a large momentum.

2. A bullet of smaller mass has a large momentum because of its extremely large velocity.


The Difference Between Momentum and Inertia





Momentum is defined as the tendency of a body to remain in motion.

In simple words, momentum is your force or speed of movement.

Inertia is defined as the tendency of a body to oppose the change in its position.

In simple language, inertia is what keeps you going.


A vector quantity, having both magnitude and direction.

It is a scalar quantity.


It is denoted by the letter ‘p’.

It is denoted by the letter ‘I’.


The momentum of a body can be calculated by the formula, p = mv.

Inertia is immeasurable.


The net momentum of the system of particles remains constant.

It has nothing to do with the conservation of energy.

FAQ (Frequently Asked Questions)

Q1: What is the Law of Conservation of Momentum Example?

Ans: When a ball is thrown from the roof of a building, it traces a parabolic path. In the horizontal direction, the air resistance is negligible, because horizontal forces acting on the ball is zero; therefore, its momentum remains unchanged.

Q2: What Causes Inertia?

Ans: Inertia is an innate property by which it resists the change in its position.

If it is in a state of rest, it continues to remain at rest and if it is in linear motion, it tends to remain in linear motion.

For example, a bike cannot change its straight-line path on its own. We have to turn its handle to change its direction.

Q3: How Do You Explain Inertia?

Ans:  Inertia of the body is measured by the mass of the body.

Heavier the body, greater is the force required to change its state of rest or motion, hence greater is its inertia.

Q4: What are the Types of Momentum?

Ans: There are two types of momentum, that are:

Linear Momentum:

The product of mass and the velocity is called the linear momentum.

Angular Momentum: 

The quantity of rotation of a body; it’s the product of the moment of inertia and its angular velocity.

A spinning object has angular momentum.