What is Inertia?

What Does Inertia Mean

We explain what inertia is and what types exist. Newton's principle of inertia and everyday examples where inertia is experienced.

The seat belt overcomes the inertia of the passengers when braking or colliding.

What is inertia?

In physics inertia is called the resistance that bodies oppose to modify their state of motion or stillness , either to alter their speed, their course or to stop; although the term also applies to modifications of your physical condition.


A body thus requires force to overcome inertia to alter its trajectory , which otherwise would stick to the laws of the movement rectilinear uniform, or to start a movement, because otherwise remain idle. This, of course, considering that there is no absolute rest or rectilinear and uniform motion in the universe , except on the basis of a reference system (of observation ). That is why it is preferred to speak of "relative rest".

In this way, a body or system will have greater inertia to the extent that it requires forces of greater intensity to modify its state of movement or to modify its physical state. The "inertial forces" are fictitious forces that the observer perceives within the frame of reference.

It can help you: Acceleration

Types of inertia

Thus, two types of inertia are distinguished in physics: mechanical and thermal.

  • Mechanical inertia. Related to the difficulty of modifying movement and stillness, as we have explained previously. It depends directly on the amount of mass of the body or system and the inertia tensor.
  • Thermal inertia. It measures the difficulty of a body or system to modify its temperature when it comes into contact with other objects or when it is heated directly. It depends on the heat capacity of the body or system.

However, mechanical inertia can in turn be subdivided into:

  • Dynamic inertia. It is presented by bodies in relative motion.
  • Static inertia. It is presented by bodies at relative rest.
  • Rotational inertia. It is presented by bodies exhibiting rotational motion.
  • Translational inertia. It is linked to the total mass of the bodies.

Principle of inertia

The principle of inertia was formulated by Sir Isaac Newton.

The principle of inertia, known as Newton's First Law , states that bodies will tend to retain their state of rest or uniform rectilinear motion until an external force capable of overcoming said resistance is applied to them , which is called as previously we said, inertial force.

This principle of physics was formulated mathematically by Sir Isaac Newton in his Philosophiae naturalis principia mathematica published in 1687, based on the well-known Law of Inertia of Galileo Galilei. And one of its fundamental concepts is the equivalence between the state of rest (speed 0) and that of rectilinear and uniform motion, since in both cases, if they occur, they imply that no external force is acting on the body in question.

On the other hand, if we observe a body moving and gradually losing speed, we can attribute this loss of speed to the effect of friction forces that overcome its inertial principle.

Examples of inertia

Inertia can be verified and experienced through numerous examples. Some may be:

  • Seatbelt. When a vehicle is moving at a constant speed, its passengers share this speed with it. But if the driver suddenly stops the vehicle (or collides with another that prevents it from continuing on its path), the passengers will feel the thrust of the inertia that makes them maintain the movement they had before the stop, throwing them forward. Then the seat belt intervenes, overcomes the inertia and interrupts their movement, preventing them from hitting the windshield.
  • Pushing a heavy object. When pushing a heavy object at rest, the need is felt to overcome inertia with the force of those who push. Once defeated, the object will move more easily, as it will be in motion; but initially it will resist moving.
  • Quickly pull a tablecloth. In the typical act of magicians, a tablecloth is pulled with objects on top, which remain in place due to inertial forces and do not move along with the cloth.
  • Braking of trains. When trains seek to stop at the station, they take a while to do so, since the inertia they bring is so high that they require more braking space .
  • The adobe of the constructions. Adobe is a common building material, especially in the most precarious homes, because it has a great thermal inertia: it resists heating, keeping the interior of the home cooler.
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