Английский язык. Ч.3 (Units 9,10). Ильичева Н.А - 32 стр.

set of coordinate axes fixed to some body such as the Earth, a train, the Moon
and so on. When we say that we observe or make measurements from a certain
reference frame, it means that we are at rest in that reference frame. It is easiest
to analyze events when they are observed and measured from inertial frames.
The Earth, though not quite an inertial frame (it rotates), is close enough and for
most purposes we can consider it an inertial frame. Rotating or otherwise
accelerating frames of reference are noninertial frames. On a rotating platform
(say a merry-go-round), for example, an object at rest starts moving outward
even though no body exerts a force on it. This is therefore not an inertial frame.
Einstein dealt with such complicated frames of reference in his general theory of
relativity.


(2;…)
     Both Galileo and Newton were aware of what we now call the relativity
principle applied to mechanics: that the basic laws of physics are the same in all
internal frames. You may have recognized its validity in everyday life. For
example, objects move in the same way in a smoothly moving (constant
velocity) train or airplane as they do on Earth. (This assumes no vibrations or
rocking – for they would make the reference frame noninertial.) When you walk,
drink a cup of soup, play Ping-Pong, or drop a pencil on the floor while
traveling in a train, airplane, or ship moving at constant velocity, the bodies
move just as they do when you are at rest on Earth. Galilean-Newtonian
relativity involves certain unprovable assumptions that make sense from
everyday experience. It is assumed that the lengths of objects are the same in
one reference frame as in another, and that time passes at the same rate in
different reference frame. In classical mechanics, then, space and time are
considered to be absolute: their measurement doesn’t change from one reference
frame to another. The mass of an object, as well as all forces, are assumed to be
unchanged by a change in inertial reference frame. That the laws of mechanics
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