# Inertia

Inertia in Latin means "peace", "inaction". Inertia, or inertia, is the desire of the body to keep its state unchanged with respect to the inertial (in the first approximation fixed) frame of reference. That is, if no external forces act on the body (applied from other bodies and the environment in general) or if these forces balance each other, the body maintains a state of rest or uniform rectilinear motion (and this in mechanics is the same as rest). If an unbalanced system of external forces acts on the body, it gradually begins to change its speed.

We all know "tricks", which "demonstrates" the inertia. If you sharply pull out the tablecloth, then the objects on it do not fall. Particularly impressive is the experience where a heavy object - a load - is suspended on a thread, and another thread hangs from it, and if desired, you can tear any of them - either the one on which the object is suspended or hanging down. If you sharply pull the hanging thread, the inertia of the load will not allow it to accelerate and the hanging thread will break. If we pull slowly, then to the force of gravity of the load, the force will be added, with which we pull downwards, and the upper thread breaks: inertia in this case "helps" very little because of the "static" tension of the threads, when the speed of the load varies very slowly.

There is inertia in the so-called inertial frame of reference. Earlier we said that in the first approximation this is a fixed system. But there is nothing absolutely immobile in the world - everything moves relative to each other. The Earth rotates around its axis and around the Sun, not to mention the perturbations of its rotation due to the motion of other planets. The sun is moving relative to the center of the Galaxy, Galaxy is scattered relatively the center of the world, which... and so on.

If our reference frame moves unevenly or indirectly with respect to the inertial system, then it can not be inertial, since it will no longer observe the law of inertia, the inertia of massive bodies will not be manifested, and consequently the laws of motion and conservation will lose their force - basic laws of mechanics. This will happen because the material point placed in a non-inertial system will have an acceleration even in the absence of external acting forces, since even without them it will participate in the accelerated forward or rotational motion of the reference frame itself.

Thus, if we want to have a very precise (absolute - unattainable!) Inertial frame of reference, we should place its center in the center of the Sun - more precisely, at the center of mass Solar System, and point the axes to three fixed (conditionally) stars.