Inertia :- Definition, types and Law of Inertia

On this page we will learn about law of inertia in physics, inertia and types of inertia. But before going any further on the topic we shall learn about the the Galileo's experiment on motion of objects. Let us first begin with the Galileo's experiments on the motion of objects and how he defined inertia and law of inertia.

Galileo's Experiments on the Motion of Objects

Galileo was the first to argue that objects move at a constant speed when no external force acts on them. On the basis of doing a few simple experiments, he came to this revolutionary conclusion. Let us discuss his experiments where he studied motion of objects on an inclined plane. (a) Galileo's Experiment with single inclined plane:- Galileo first investigated the motion of objects on an inclined plane, as seen in the image below.

His Observations:-

  1. The speed of an object increases as it moves down the inclined plane. This means that the object moving down the inclined plane accelerates.
  2. The object's speed reduces as it moves up the inclined plane. This means that object moving up the inclined plane decelerates. This happens due to the pull of gravity.
  3. Galileo reasoned from the previous two observations that when the plane has no slope, there should be no acceleration or deceleration. As a result, Galileo concluded that an object on a horizontal plane should move with constant velocity in a straight line path.

Galileo Experiment with single inclined plane

(b) Galileo's Experiments on two inclined planes combined together: Galileo did another experiment using a double inclined planes facing each other as shown below in the figure

Galileo Experiment with double inclined plane

His Observations:-

  1. Our first setup is shown in figure a. It is the case when the slopes of two planes are same and distance covered in rolling down one inclined plane is same as the distance covered in climbing up the other incline. When an object rolls down one inclined plane, it climbs up another. Due to friction, it nearly reaches the same height but falls short of the initial height. If friction did not exist, the object would have reached the same height as before. Under no circumstances is the final height higher than the initial height.
  2. Our second setup is shown in figure b. The slope of the upwardly inclined plane is reduced in this setup. The object must now go a greater distance to reach its maximum height. The object would need to travel a greater distance to achieve the same height as we decreased the upward inclined plane's slope.
  3. When the slope of second inclined plane is made zero i.e., second plane is made horizontal, the ball travels infinite distance in an ideal situation when there is no friction.

What is the Law of Inertia?

Galileo derived the law of inertia from the set of experiments mentioned in the previous section. Law of Inertia states that

A body moving with a certain speed along a straight path will continue to move with the same speed along the same straight path in the absence of external forces.

We've discussed the Aristotle's Fallacy , which states that an external force is always required to keep a body moving. This was proven incorrect when the concept of inertia was introduced.

What is Inertia in physics

  • According to the first law of motion, an object at rest will not move unless it is acted on by a force.
  • Inertia of rest refers to the inherent property of objects to remain at rest unless acted upon by a force. Consider the case of an object moving in a straight line with uniform velocity. According to Newton's law, it would continue to move in a uniform manner.
  • This inherent property that causes a body in uniform motion to tend to maintain its uniform motion is known as inertia of motion.
  • Combining two statements, Inertia is the property of an object to remain at rest or in uniform rectilinear motion unless acted upon by a force.

Relation between mass and inertia

  • The mass of any body is a measure of its inertia. For example, if we apply an equal amount of force to two objects of different mass (say, say, $m_1$ and $m_2$), the acceleration of both objects will be different (i.e., both $a_1$ and $a_2$ would be different).
  • The acceleration of a larger mass object would be less than the acceleration of a smaller mass object. As a result, as the body's mass increases, the acceleration decreases and the inertia increases.
  • In other words, the heavier the body, the greater the force required to change its state, and thus the greater its inertia. The opposite is also true.
  • Newton's first law of motion, also reveals this fundamental property of matter (inertia) and is also referred to as the law of inertia.

Types of Inertia

Inertia of a body are of three types

  1. Inertia of rest
  2. Inertia of motion
  3. Inertia of direction Let us discuss each one of these types of inertia along wit few examples.

Inertia of Rest

It is a body's tendency to remain in its resting position. This means that a body at rest cannot begin to move on its own. Rather, a body resists the force attempting to move it.

Examples of Inertia of rest
  1. A person standing in a bus falls backwards when the bus suddenly starts moving forward. This happens because when the bus moves, the lower part of his body moves with it, but the upper part of his body stays still because of inertia. So, when the bus starts, the person falls backwards.
  2. Due to the inertia of the rest of the upper body, the rider falls backwards when the horse suddenly starts to move. This can be explained using the argument given in the previous example.
  3. When a carpet is pounded with a stick, dust particles fall out. This is due to the fact that the beating causes the carpet to move, whilst the dust particles tend to remain at rest and therefore separated.

Inertia of Motion

The tendency of body to remain in state of uniform motion in a straight line is called inertia of motion. This means that a body in uniform motion can neither accelerate not decelerate on its own and come to rest. As a result, a body in uniform motion opposes the force trying to stop it.

Examples of Inertia of Motion
  1. When a moving bus comes to a sudden stop, a rider falls forward. As the bus comes to a stop, the lower part of the rider's body comes to rest with it, while the upper part of the rider's body continues to move due to inertia of motion and falls forward.
  2. A person who jumps out of a moving train may fall forward. This is due to the fact that his feet come to rest on touching the ground, while the rest of his body continues to move due to inertia of motion.

Inertia of Direction

Inertia of direction is the inability of the body to change by itself its direction of motion. This implies that a body keeps moving in a straight line unless forced to do otherwise by outside forces. As a result, a body resists the force that attempts to change its direction of motion.

Examples of Inertia of Direction
  1. When a bus makes a sharp turn, passengers feel a force acting away from the center of the curving path. This occurs as a result of the tendency to move in the original direction. To keep from swaying away in the turning bus, the person must grab on to a support.
  2. The mud from a moving vehicle's wheels flies off tangentially. This happens due to directional inertial.
  3. It is recommended that we use a rope to secure our bags on the roof of a bus. Because of the inertia of direction, bags might be thrown sideways when the bus makes a sharp turn.
Further References
  1. Galileo's Acceleration Experiment (

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