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was introduced by Newton as a measure of inertia.
Mechanical motion is relative and its character for the same body can
be different in different frames of reference .
Newton's first law is not fulfilled in all frames of reference but only

in inertial frames of reference.
Inertial frame of reference is such frame of reference regarding to
which a body free of external force remains in its state of rest or of

motion at uniform velocity in a straight line.
Newton's first law includes two statements:1) all matter possesses
inertia and 2) there exist inertial frames of reference. All laws of physics
are the same in all inertial frames of reference .

3.2. Mass. Force. Newton's Second Law. Impulse

Mass of a material point (body, object) is a scalar value that is a
measure of its inertia. Mass is the quantity of matter a body contains, as
measured by acceleration under a given force. On this basis the standard

unit of mass is kilogram (kg).
The inertia of an object depends on its mass. The greater the inertia
of an object, the greater is its mass. Under the action of external force an

object changes its velocity not instantly but gradually, little by little
possessing certain acceleration, namely
1
a ~
m (3.1)

To compare two masses it is enough to measure modulus of acceleration:
m m
 2
m 2 m 1 . (3.2)

If m and m m interact, we obtain
1
2
m  a  m a 
1 1 2 2
(3.3)
In dynamics a new concept to describe the interaction between the
objects must be introduced. This is the concept of force F. The word

"force" was used repeatedly in the discussion above, even though it had
not been previously defined.
As follows from Newton's first law force is defined as value
which is necessary to accelerate an object. Forces may vary in

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