Inertia and its Types: Definition, Law of Inertia, History

Inertia is the property of any object or system that resists any change in its state of rest or motion. Let us learn about types of inertia here. Every day we see a lot of events happening all around us that are a direct consequence of inertia. What happens when a strong wind shakes a tree? Fruits and leaves fall. We feel a force in the direction of motion of the bus. What happens if brakes are suddenly applied in an accelerating car? All the passengers feel a forward push.

Similarly, what would happen if a rapidly accelerating car suddenly makes a right turn? All the passengers will feel a push towards the left. What is the reason behind this invisible push or pull? It is Inertia. Qualitatively speaking, inertia is the property of any object that tends to keep them undisturbed; this means that the stationary objects remain at rest or the objects in motion continue moving along their path with the same velocity.

What is Inertia?

Inertia is the tendency of an object to resist any change in its state of rest or motion. In other words, inertia is the inherent property of a body that tries to maintain its state. If left undisturbed, a body in a state of rest would continue to be at rest, and a body in a state of uniform motion will keep moving the same way unless it is acted upon by an external and imbalanced force. With the help of inertia, it was proved that Aristotle’s view that an external force is required to keep a body in uniform motion was, in fact, wrong. Force is only needed to change the state of a given body.

CLEAR YOUR CONCEPTUAL DOUBTS ON INERTIA

First Law of Motion – Law of Inertia

According to newton’s first law of motion, “An object at rest stays at rest, and an object in motion stays in motion with the same speed and the same direction unless acted upon by an external and unbalanced force.” Or, in simpler terms, it states that if left undisturbed, bodies tend to “keep on doing what they’re doing”. Every object in this world has a natural tendency to resist a change in its state; it can be its state of rest or its state of motion.

Before his theory was conceptualized, many scientists before Newton thought that all bodies have the inherent tendency to achieve the state of rest. It was believed that all the bodies in motion would eventually come to rest if the force acting on them was removed. Or, in other words, an unbalanced force was necessary to keep an object moving, and in the absence of that force, a body would eventually stop moving and come at rest. Thus, a moving object will ultimately come to rest in lack of force, and an object at rest will remain at rest only. This idea was in direct opposition to the concept of Inertia that Newton proposed years later.

History of the Concept of Inertia

Although inertia has been in theories of various philosophers and scientists over the last \(1000\) years and more, Galileo is credited with conceptualizing inertia. This seventeenth-century scientist suggested that all the moving objects achieve rest over time due to dissipative forces like friction. To prove his hypothesis, Galileo conducted several experiments. First, he studied the motion of marble over several inclined planes.

1. As the marble was released, he noticed that as the marble rolled down on the inclined plane, it sped up or its velocity increased, but it slowed down as it went over the inclined plane.
2. Galileo observed that when the marble was thrown down a smooth inclined plane, It was found that the marble rolled up the same height, along the plane equal to the height that it rolled down.
3. After studying the marble’s motion across the inclined planes, he concluded that the marble would cover an equal distance up and down the plane for the same height of inclined planes on both sides.
4. Also, he observed that when the angle of inclination of the right-side plane was gradually decreased, the marble would travel further distances until it travelled the same distance as it would have to reach the original height.
5. When the slope of the right-side plane decreased so that it becomes equal to zero, i.e., it was ultimately made horizontal, the marble, trying to reach the same height that it was released, will move forever. This is because the unbalanced forces on the marble, in this case, are zero. Thus, the marble will not stop.
6. The results from these experiments suggested that an unbalanced (external) force is required to change the marble’s motion, but no net force is needed to sustain the uniform motion of the marble.

Do we Need a Force to Keep an Object Moving?

Sir Isaac Newton further worked on Galileo’s ideas about motion, and he proposed the three famous laws of motion. Newton’s first law of motion suggested that a force is not needed to keep an object in motion, it is only needed to accelerate an object. To understand this, take a box and slide it across a table and leave it be. Eventually, the box will slide to a rest position.

This sliding (moving) box on the table does not achieve the rest state because of the lack of a force, but instead, the presence of an invisible force brings the box to a halt. This force is the force of friction – that brings the box to a rest position. If there were no friction force between the box and the table, the box would continue to slide over the table with the same speed and direction – forever! (Or at least to the end of the table.) Likewise, a force is not required to keep a moving book in motion. However, a force is required to bring the book to rest.

Types of Inertia

Inertia is the inherent property of a body by virtue cannot change its state of rest or uniform motion unless a force acts on it. There are three types of inertia:

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

1. The inertia of rest: According to the inertia of rest, a body in the state of rest continues to rest unless an external unbalanced force acts upon it. Thus, a body at rest will resist a change in its state of rest.

Examples:
a. When the bus driver starts the bus suddenly, a person standing in the bus feels a jerk and falls backwards. This happens due to the inertia of rest. The person’s feet, while he was standing in a stationary bus, are at rest. As the bus starts moving, the person’s body starts to move along the direction of the bus, while his feet continue to be at rest. That’s when he feels the jerk, and he falls back.
b. When a horse starts running suddenly, the rider on the horse falls backwards.
c. When we place a coin on a smooth piece of cardboard covering a glass and strike the cardboard piece with a finger suddenly, the cardboard slips away, and the coin falls into the glass.

2. The inertia of motion: According to the inertia of motion, a body in the state of uniform motion continues to move with the uniform speed in the same direction unless an external unbalanced force causes it to change its present state.

Examples:
a. Men in space find it more difficult to stop moving because of a lack of gravity acting against them.
b. When a player is tackled while playing football and his head strikes the ground, his skull comes to rest with the impact, but due to inertia of motion, his brain remains in motion, and it often gets injured.
c. To prevent our bodies from undue injury in a car accident, we are advised to wear seatbelts. When brakes are suddenly applied, our body feels a jerk in the forward direction and to prevent his forward motion to cause any fatal injuries. Seatbelts are provided so that the person remains in his position and thus, can avoid injury.

3. The inertia of direction: According to the inertia of direction, a body moving in a given direction will continue to move along the same direction unless external forces cause it to change its direction.

Examples:
a. Hovercraft can be a challenge to manipulate because, unlike cars, they do not have the same level of friction. Hence, inertia causes the hovercraft to want to continue in the same direction without stopping or turning.
b. The sudden turning of a shopping cart can cause the objects kept in the cart to tumble down because of the inertia of direction.
c. If you jump from a car or bus that is moving, your body is still moving in the vehicle’s direction. As soon as your feet come in contact with the ground, they come at rest while your upper body stays in motion. This causes our body to fall in the forward direction.

Inertia and Mass

All objects resist any change in their state of rest or motion. All objects have this tendency which we call inertia. But, is the tendency to resist any change, the same for all objects? And if it is different, then on what factors does it depend? No, the inertia of all the objects is not the same. It depends on the mass of the object. Hence, the tendency of an object to resist changes in its state of rest or motion varies with mass. The more the mass of an object, more the inertia and vice versa. The greater the mass of an object, the larger the force will be required to change its state. Thus, a more massive object has a greater tendency to resist changes in its state of rest or motion.

Suppose two identical balls are kept on a horizontal floor. One ball is made of steel, and the other ball is made of rubber. It is told that you have to tell which one is which without lifting them. How will you do it? By employing the knowledge of inertia of rest. A body in the state of rest would continue to do so unless acted upon by an external force. Now You could give the balls an identical push to change their state of motion. The ball that offers the least resistance is the ball with the least inertia – and therefore, it will be the ball with the least mass (i.e., the rubber ball).

Summary

Inertia is the tendency of an object to resist any change in its state of rest or motion. In other words, inertia is the inherent property of a body that tries to maintain its state. Newton’s first law of motion-defined inertia.

According to newton’s first law of motion, “An object at rest stays at rest, and an object in motion stays in motion with the same speed and the same direction unless acted upon by an external and unbalanced force.”

The results from Galileo’s experiments suggested that an unbalanced (external) force is required to change the marble’s motion. Still, no net force is needed to sustain the uniform motion of the marble.

There are three types of inertia:

1. Inertia of rest: According to the inertia of rest, a body in the state of rest continues to rest unless an external unbalanced force acts upon it.
2. Inertia of motion: According to the inertia of motion, a body in the state of uniform motion continues to move with the uniform speed in the same direction unless an external unbalanced force causes it to change its present state.
3. Inertia of direction: According to the inertia of direction, a body moving in a given direction will continue to move along the same direction unless external forces cause it to change its direction.

The tendency of an object to resist changes in its state of rest or motion varies with mass. The more the mass of an object, more the inertia and vice versa.

PRACTICE QUESTIONS ON INERTIA

FAQs

Q.1. What is Inertia?
Ans: The property of an object by which it cannot change its state of rest or uniform motion along a straight line on its own is called inertia.
Inertia is a measure of the mass of a body. Thus, the greater the mass of a body greater will be its inertia or vice-versa.

Q.2. How many types of inertia are there?
Ans: Inertia is defined as “the tendency of an object to stay at rest or in motion”. There are three types of inertia:
a. Inertia of rest
b. The inertia of motion
c. Inertia of direction

Q.3. What is the inertia of motion? Give an example.
Ans: The property of a body to remain in its state of uniform motion along a straight line is called Inertia of Motion.
Example:
a. When a running horse stops suddenly, the rider often falls forward.
b. A long jump athlete often runs a long distance before jumping.

Q.4. Consider the following scenario: A person sitting in a car falls backwards when the car starts. What type of inertia does it represent?
Ans: This example represents the case of inertia of rest. As the lower portion of the car comes in motion, the upper part tries to maintain its state of rest because of inertia.

Q.5. Which newton’s law explains inertia?
Ans: Newton’s first law of motion defines inertia.  The first law of motion states that a body maintains its state of rest or of uniform motion along the same straight line unless an external force acts on it, causing it to change its state.

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