Inertia: Definition and Types with Examples

In this article, we will learn about Inertia, its definition, and types of inertia like the inertia of rest, motion, and direction along with the examples of each type of inertia.

Understanding the Principle of Inertia

Ancient Views on Motion

Long ago, philosophers like Aristotle thought that objects moved only when they interacted with other objects. They believed that an object’s natural state was to be at rest and that it needed an external force, or push, to move. This idea was hard to explain when they considered things like a thrown stone or a flying arrow, which seemed to keep moving even when nothing was pushing them.

Revisiting the Idea of Motion

In the late 14th century, two things made people rethink motion. First, people needed to understand how cannon balls moved because of the development of artillery. Second, Nicolaus Copernicus proposed that the Earth was not the center of the universe, but just one of many planets moving around the sun. This made people realize that objects could move on their own, without being continuously pushed by something else.

Galileo and Newton’s Contributions

Galileo Galilei, in the early 17th century, was the first to suggest that if nothing was acting on an object, it would keep moving forever. This idea was later refined by Sir Isaac Newton in his first law of motion, also known as the law of inertia. According to this law, if nothing is interacting with an object, it will keep moving in a straight line at a constant speed. This is called inertial motion.

What is Inertia

Inertia is described as a physical object’s resistance to changes in speed and state of rest.  It keeps an object moving in the same direction unless acted upon by another force. The more inertia, the greater the resistance to acceleration or deceleration. The word inertia comes from Latin meaning “to remain”.

Law of Inertia

Newton’s first law of motion states that a body remains in a state of rest or uniform motion unless and until it is acted on by an external force to change its state. 

See also Rest and Motion in Physics

This means a body, on its own, can not change its state of rest or state of uniform motion along a straight line.

Unit of Inertia

In physics, the concept of inertia refers to the resistance of an object to any change in its velocity, which includes changes to both speed and direction. It’s a fundamental principle that underlies the motion of objects.

However, when we talk about the “unit of inertia,” it’s important to note that inertia itself doesn’t have a specific unit. This is because inertia is a property of matter and is not a physical quantity that can be measured in terms of units like meters, kilograms, or seconds.

That said, we often discuss related quantities that do have units. For example, the “moment of inertia” is a measure of an object’s resistance to changes in its rotational motion. The unit of moment of inertia in the International System of Units (SI) is the kilogram meter squared (kg·m²). This is derived from the formula for the moment of inertia, I = Σmr², where m is the mass of a point in the object, and r is the distance of that point from the axis of rotation.

So, while inertia itself doesn’t have a unit, related quantities like the moment of inertia do, and these can be very important in physics, particularly in the study of rotational motion.

Types of Inertia

As we have discussed, inertia is the inability to change its state unless we apply an external force to the body. We can categorize inertia into three types

1. Inertia of rest
2. inertia of motion
3. Inertia of direction

We will now discuss these three types of inertia one by one.

Inertia of rest

The inertia of rest is the inability to change its state of rest. This means that if a body is at rest, it remains at rest and can not start to move on its own. It would rather oppose the force that tries to bring it to motion.

Example of Inertia of rest

1. When a bus driver suddenly starts the bus we get thrown backward with a jerk. This is due to their bodies’ inertia, which causes the passengers to remain in a state of rest even after the bus has started moving. 
2. When a horse starts suddenly the rider tends to wall backward on account of the inertia of the rest of the upper body. This happens because the lower part of the body which is in contact with the horse comes into motion while the upper part tends to remain at rest due to inertia.
3. When we shake a tree the leaves fall. This is because the branch comes in motion and leaves tend to remain at rest. Hence they get detached.
4. Dust is removed from a hanging carpet by beating it with a stick. As the carpet is beaten, it is abruptly pushed forward, but the dust particles tend to remain at rest due to inertia and fall off.
5. A tablecloth can be yanked out from beneath the dishes with a quick pull. As long as the friction from the tablecloth movement isn’t too great, the dishes will tend to stay still.
6. It is preferable to remove a Band-Aid as quickly as possible. Because of inertia, your skin remains at rest, and the force pulls off the Band-Aid.

Inertia of motion

The inertia of motion is the inability of a body to change by itself its state of uniform motion. This means if a body is in the state of motion then it can not come to the state of rest or accelerate on its own. Rather a body in uniform motion opposes the force that tries to stop it.

Examples of Inertia of motion

1. When a moving bus stops suddenly or a driver applied breaks suddenly then we are thrown forward with a jerk. This happens because If the bus suddenly stops, the passengers continue to move ahead at the original speed of the bus. 
2. When a horse running fast stops suddenly, the rider is thrown forward if he is not seated firmly. Since the rider’s lower body is in contact with the horse, the lower body moves. On the other hand, due to inertia, his upper body remains stationary.
3. A person jumping out of a moving train falls in the forward direction. This is because his feet come to rest on touching the ground and the remaining body continues to move due to inertia of motion.
4. An athlete runs for a certain distance before taking a long jump. The inertia of motion gained by him adds to his muscular effort. Thus at the time of jumping it helps him in taking a long jump.
5. If you hit something while rolling a cart with something on top and it stops, the item on top may fall off.
6. Stopping a bus is more difficult than stopping a motorcycle. The larger object has more inertia.

Inertia of Direction

The inertia of direction is the inability of a body to change by itself its direction of motion. This means that a body continues to move along the same straight line unless it is compelled by external forces to change its course of motion.

The body under consideration would rather oppose the force that tries to change its direction of motion.

Examples of Inertia of Direction

1. An umbrella protects us from the rain and sunlight. It is based on the property of inertia of direction. When the umbrella is up, raindrops falling straight down cannot change course and cannot wet us.
2. When a car makes a round turn, the occupants within are pushed in an outward direction. It occurs as a result of the inertia of direction in order to retain the motion’s direction.
3. The rotating wheels of any vehicle throw out mud if any, tangentially. This happens due to the inertia of direction. Due to this very reason wheels of various vehicles have mudguards.
4. When a knife is sharpened by pressing it against a grinding stone, the sparks fly off along the tangent to the grinding stone. This happens on account of directional inertia.

Frequently Asked Questions on Inertia