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Difference Between Momentum and Inertia

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Last updated date: 24th Feb 2024
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In this particular article we shall be learning about the following concepts - 

  • An introduction

  • What is Inertia?

  • Types of Inertia

  • Examples of Inertia 

  • What is Momentum with examples

  • Difference Between Momentum and Inertia

  • Frequently asked questions


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:

  • 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

  • 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

  • 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 a 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 a smaller mass has a large momentum because of its extremely large velocity.

 

The Difference Between Momentum and Inertia

S.No.

Momentum

Inertia

1.

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.

2.

A vector quantity, having both magnitude and direction.

It is a scalar quantity.

3. 

It is denoted by the letter ‘p’.

It is denoted by the letter ‘I’.

4.

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

Inertia is immeasurable.

5.

The net momentum of the system of particles remains constant.

It has nothing to do with the conservation of energy.


Key learnings from the chapter - 

  • Inertia is based on Newton’s first law of motion. 

  • A body remains at rest until a force is applied to change its position

  • In Inertia of direction, the body moves in the linear path until a force is applied to change its direction

  • Momentum is the product of mass and velocity. The force of momentum is directly proportional to the mass and velocity. Larger the mass, the more the momentum. Similarly, more the velocity means higher momentum

  • Thus, momentum is the tendency of an object to remain in motion while the inertia of an object has the tendency to oppose the changes.

FAQs on Difference Between Momentum and Inertia

1.What are the different types of momentum?

There are two different types of momentum, they are - 

  • Angular Momentum - it is obtained by multiplying a body's mass by its angular velocity. 

  • Linear Momentum - is the quantity of mass related to a body that moves along a straight path.

2.What is the importance of learning the topic momentum?

Momentum is a very significant concept of physics as it describes the relationship between speed, mass, and direction. The concept of momentum is essential in sports, business, studies, even in your day-to-day activities like walking, eating, running, etc. It is the most important part of human activity.

3.How Do You Explain Inertia?

In simple words, inertia states the object remains to be in steady or in motion until an external force is applied to it. Inertia is the property of an object or a body that resists changes to the state of the motion and which varies with the mass. As inertia is measured by the mass of the object, the heavier the object the more is the force required to change its motion. Hence, more is the inertia of that object or body.  


For example - a bike cannot change its straight path until an external force is applied to change its position.

4.What are some examples of inertia?

Inertia is the resistance of an object against the change in its motion. Some of the examples are - The inertia of the motion keeps the satellite moving around the larger object. Another example is the dust particles from a carpet. The initial inertia of rest makes the particles stay at rest, the frequent beating of the carpet results in the removal of the dust particles.

5.What is the Law of Conservation of Momentum Examples?

According to this law a momentum can never be created or destroyed, as objects in isolation cannot change their motion and it requires an external force. 


For example - a ball when thrown from the roof of a building moves in a parabolic path. So, when the ball moves in a horizontal direction, there is no external force as the force of air is negligible upon the ball. Thus, the momentum of the ball remains unchanged.

6.What Causes Inertia?

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.

7.How Do You Explain Inertia?

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.

8.What are the Types of Momentum?

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.