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is based on the principle that all uniform motions are relative and that light
has constant velocity in vacuum, regarding space-time as four-dimensional
continuum, and modifying previous conceptions of geometry. General
theory of relativity is extending this to gravitation and accelerated motion.

Quantum mechanics (QM – also known as quantum physics, or
quantum theory) deals with physical phenomena at microscopic scales,
where the action is on the order of the Planck constant. Quantum

mechanics departs from classical mechanics primarily at the quantum
realm of atomic and subatomic length scales.
Depending on the conditions of certain problem different simplifying
models are used in mechanics to describe the motion of real objects.

Material point is a body for which its shape and size are not
significant in the certain problem.
Absolutely (perfectly) rigid body is an object for which the distance

between any of its two points is always constant.
Absolutely (perfectly) elastic body is an object for which its
deformations are described by the Hook's law, namely are proportional

to forces, which cause them.
Absolutely (perfectly) non-elastic body is an object, which keeps its
deformed state even when an external force is ceased.

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