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is obtained which shows how much the first value differs from the second
one.
A numerical meaning of any physical value depends on the
system of units.

The 1960 General Conference of the International Bureau of Weights
Measures established the International System of Units (SI) by selecting
the meter as the basic unit for length, the kilogram for mass, the second

for time, the ampere for current, the kelvin for temperature, the candela
for luminous intensity, and the mole for quantity of substance.
The meter is the length of 1 650 763,73 times the wavelength of the
orange-red light given off by the pure krypton isotope of mass 86 when it is

excited in an electric discharge.
The second is the duration of 9 192 631 770 periods of the radiation
corresponding to the transition between the two hyperfine levels of the

fundamental state of the atom of cesium 133.
One of the basic concepts of physics is the concept of motion. Lack
of understanding of how and why motion takes place hampered the

development of many branches of science for many years. Some of
concepts were eventually clarified during the sixteenth and seventeenth
centuries mainly due to the works of Galileo Galilei (1564-1642) and

Isaac Newton (1642-1727).
The simplest form of the motion in nature is a mechanical motion
— an alteration of relative space positions of bodies or their parts.
The word mechanics is derived from the Greek mechanics.

Mechanics is the part of physics that deals with the way forces affect still or
moving bodies. Mechanics deals with studying the objective laws of
mechanical motion.

The system of studying motion divides mechanics into three
branches: kinematics, which deals with motion without considering its
cause, dynamics, which considers both motion and its cause (forces), and
static, which deals with laws of addition of forces and conditions of

equilibrium of bodies.
Mechanics consists of classic mechanics , relativistic mechanics and
quantum mechanics

Classic (Newton's) mechanics deals with the motion of macroscopic
bodies with low velocities (much less than the velocity of light in
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vacuum c = 3 10 m/s).
Relativistic mechanics deals with the motion of bodies with velocities
compared with the velocity of light in vacuum. Special theory of relativity

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