Inertial Force

All About Inertia

Physics is technically analogous to the contributions of Sir Isaac Newton. He is the man who revolutionized classical physics with his laws of motion. He propounded three laws of motion, and the first of these is related to inertia. But first, let us first understand the meaning of inertia. 

The term 'inertia' comes from the Latin word 'iners', which translates to lazy or idle. Johannes Kepler coined the term. The meaning of inertia is related to the fixed characteristic of an object made of matter. Inertia is a quality found in all things made of matter that have mass. An object made of matter keeps doing what it is doing until there is a force that changes its speed or direction. A ball on a table will not start rolling unless someone or something pushes it. It is noteworthy that if you toss a ball in a frictionless vacuum space, the ball will keep moving at the same speed and direction forever unless there is some action created by gravity or collision. The measure of inertia is mass. Objects with a greater mass resist a change in their motion or rest more than objects with lower mass. For example, moving a truck will require more forceful pushes. On the contrary, moving a bike will require less aggressive impulses. This difference in force is because the truck and bike have different masses. A truck has more significant inertia than that of a motorcycle.

Definition of Inertia

We can define inertia as the property of an object by which it cannot change its state of rest along a straight line on its own unless acted upon by an external force. Inertia increases with an increase in the mass of the body and vice-versa. We experience a jerk while suddenly using the brakes of a moving car because of inertia.

The Law of Inertia

Sir Isaac Newton utilized and expanded the principles shown in Galileo's observations into his first law of motion. He gathered that it requires force for a moving ball to stop rolling once it is in motion. It takes force to change the ball's speed and direction. In Newton's Principia Mathematica, he defined the Law of Inertia as "the motion of bodies included in a given space are the same among themselves, whether that space is at rest or moves uniformly forwards in a straight line without circular motion."

Thus, Newton's first law of motion asserts that an object will continue to be in the state of rest or a state of motion unless an external force acts on it. 

Newton's Second Law of Motion defines the relationship between acceleration, force, and mass.

Newton's Third Law of Motion states that an equal force acts back on the original object any time a force acts from one thing to another. This law means that every action has an equal and opposite reaction. If you pull on a rope, therefore, the rope is pulling back on you as well.

Fictitious Force

A fictitious force acts on all masses whose motion we can describe using a non-inertial frame of reference. Fictitious force comes in effect when the frame of reference has started acceleration compared to a non-accelerating frame.

This force arises when there is no physical interaction between two objects. But, instead, the acceleration of the non-inertial reference frame leads to the formation of fictitious force. On account of the arbitrary nature of a reference frame, the fictitious force can also be arbitrary. The leading fictitious forces are the Centrifugal force, Coriolis force and Euler force. Fictitious force is also known as Inertial force or Pseudo force.

We can understand the fictitious force with an example. If a person standing at a bus stop is watching an accelerating car, he infers that a force is exerted on the vehicle. Hence, there is no fictitious force in this scenario. But, if the person inside the moving car is looking at the person standing at the bus stop, he realizes that person is accelerating with respect to the car, although no force is acting on it. Here, the concept of fictitious force is necessary to convert the non-inertial or still frame of reference to an equal inertial frame of reference.

Types of Inertia

The inertia of Rest refers to the inability of a body to change its state by itself. For example, when we shake the branches of a tree, the leaves fall because the components they are attached to come into motion. On the other hand, the leaves tend to be at rest and hence, get detached.

The inertia of motion refers to the inability of a body to change its state of uniform motion by itself. For example, when a moving car suddenly stops, the person sitting in the car falls forward because the lower portion of the body contact with the vehicle comes to rest. In contrast, the upper part tends to remain in motion due to the inertia of motion.

The inertia of direction implies that the body cannot change its direction of movement by itself. For example, when a car takes a curve, the person sitting inside is thrown outwards to maintain his direction of motion. This phenomenon happens due to the inertia of direction.

Formula of Inertia

We can understand the moment of inertia as a quantity that decides the amount of torque needed for a specific angular acceleration in a rotational axis. The moment of inertia is alternatively called angular mass, and its SI is kg.m2.

In General form, we can express the Moment of Inertia in the following way 

I = m x r2


m = sum of the product of mass.

r = distance from the axis of rotation

I = Integral form 

M1 L2 T0 gives the dimensional formula of the moment of inertia.

The moment of inertia is the calculation of the resistance of a body required to bring change in its rotational motion. It is constant for a particular rigid frame and a specific axis of rotation.


The concept of inertia is one of the most critical topics in Physics. Understanding inertia may seem challenging. But, the topic becomes manageable with regular revision and thorough understanding. You can take the help of Vedantu's concept pages to get a firm grasp on this complicated subject.