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?

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

Such a body requires a force that overcomes inertia to alter its trajectory, which would otherwise adhere to the laws of movement uniform rectilinear, or to initiate a movement, otherwise it would remain at rest. This, of course, considering that there is no absolute rest or rectilinear and uniform motion in the universe, except based on a reference system (from observation). That is why it is preferred to speak of "relative rest".

In this way, a body o system It will have a 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 forces fictitious that the observer perceives within the frame of reference.

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 directly depends on the amount of mass of the body or system and of the inertia tensor.
  • Thermal inertia. Measures the difficulty of a body or system to modify its temperature by coming into contact with other objects or by being 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.

Inertia principle

The principle of inertia was formulated by Sir Isaac Newton.

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

This principle of physics was formulated mathematically by Sir Isaac Newton in his work 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 travel and gradually lose 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 its trajectory), the passengers will feel the push of the inertia that makes them maintain the movement they had before the stop, throwing them forward. Then the seat belt intervenes, overcomes 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 a greater space braking.
  • 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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