Difference Between Inertia and Mass

Inertia vs Mass

Mass and inertia are two concepts discussed in the field of mechanics, in physics. The concepts of mass and inertia are widely used in almost every field that has even the slightest usage of physics. Mass is a non-intuitive physical quantity of an object; inertia is also such a concept. It is vital to have a good understanding in the concepts of mass and inertia in order to excel in fields such as mechanics, relativity etc. In this article, we are going to discuss what mass and inertia are, their definitions, similarities, applications, and finally the differences between mass and inertia.


Mass is divided into three different types as inertial mass, active gravitational mass and passive gravitational mass. Experimental data shows that all three of these quantities are the same. Matter and energy are two forms of mass. The mass is measured in kilograms. The common misconception is that the weight is measured in kilograms, but the weight is actually measured in Newton. The weight is the amount of force acting on the mass. The kinetic energy of a body, the momentum of a body, and the amount of acceleration due to a force applied are dependent on the mass of the body. Apart from the day-to-day materials, things such as electromagnetic waves also have a mass.

In relativity, there are two types of mass defined as rest mass and relativistic mass. The mass of an object doesn’t remain constant throughout a movement. The rest mass is the mass measured when the object is at rest. The relativistic mass is measured for a moving object. These two are nearly the same for speeds much less than the speed of light, but varies greatly when the speed approaches the speed of light. The rest mass of the electromagnetic waves is zero.


Inertia is derived from the Latin word “iners”, which means idle or lazy. Inertia is a measurement of how lazy the system is. Inertia of a system tells us how hard it is to change the current state of the system. The higher the inertia of a system harder it is to change the velocity, acceleration, direction of the system. Objects having higher masses have higher inertia. That’s why they are hard to move. Given that it is on a frictionless surface, a moving higher mass object would be hard to stop too. Newton’s first law gives a very good idea about inertia of a system. It states “an object not subject to any net external force moves at a constant velocity”. This tells us that the property of an object does not change unless there is an external force acting upon it.

An object at rest can also be considered as an object having null velocity. In relativity, the inertia of an object tends to infinity when the speed of the object reaches the speed of light. Hence an infinite force is required to increase the current velocity. It can be proved that no mass can reach the speed of light.


What is the difference between Mass and Inertia?

• Mass is a measurable quantity, whereas inertia is a concept used to describe how hard it is to change the current state of the mass.

• For classical mechanics, mass is a property of the object itself but inertia is a property of the motion as well as the mass.

• Inertia is the concept, which is used to define the mass.