According to Newton's First Law, we know that a particle at rest (still) will remain at rest; and a moving particle will remain in motion, with constant velocity, until an external force acts on them. Therefore, this law says that an isolated particle is at rest or in uniform straight motion.
When we are stating the law of inertia, ie Newton's First Law, we have to indicate to what or to whom what is being referred to is the movement of the free-moving body. We are able to admit that the movement of the body is relative to an observer (itself), to another particle or to a free system. For the free system, the mobile does not interact with the rest of the universe.
This observer is said to be an inertial observer and the frame of reference it uses is called inertial frame of reference.
We can find different inertial observers in relative uniform motion. In this way, a free body that is at rest relative to an inertial observer can be considered in motion, having a constant speed in relation to other observers inertials.
For example, when we have a car making a curve, in relation to the Earth's surface, we can say that the car's velocity vector varies, that is, it is not constant. This means that the car making the turn cannot be considered an inertial frame of reference, as it is in accelerated motion. Therefore, we can make the following definition:
Inertial frame is a coordinate system for which Newton's First Law applies.
The Earth, due to its rotational motion, cannot be strictly considered an inertial reference. However, when we study motions of short duration, we can disregard the effects of its rotation and consider the Earth to be an inertial reference frame.
By Domitiano Marques
Graduated in Physics
Source: Brazil School - https://brasilescola.uol.com.br/fisica/referenciais-inerciais.htm
