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| gravitational field non-inertial reference frames | — — — | гравітаційне поле неінерціальна система відліку | force of inertia сentrifugal force coriolis force | — — — | сила інерції відцентрова сила сила коріоліса |
Any reference frame being in accelerated motion with respect to an inertial reference frame is non-inertial. The mechanics laws are invalid with respect to this frame of reference.
We all are aware of a strange force which appears when we drive around a curve, especially at high speed: it pushes us to the door and holds us there. This new centrifugal force acts away from the centre of the curve. But there is a real problem: how can a change in the motion of a reference system (the car) actually create a new force?
To answer this question let us consider two frames of reference: 
is an inertial frame of reference and 
moves with respect to at changing velocity 
(Fig 1.24). The position of the mass point m is determined by position vectors 
(in frame ) and 
(in ) and velocities additional rule will have form
Then for accelerations we get:
, or 
,
where 
is acceleration of material point in the system 
isacceleration of material point in the system K;
is acceleration of the system in relation to the system K.
After multiplying this equation by mass 
of the mass point we get:
,
and
,
What is the physical interpretation of the term 
? Its unit of measurement is the same as that of the force. Hence we call it a force - it is the force of inertia.
Thus, in non-inertial system of reference an additional force - force of inertia - appears. Its magnitude is proportional to the acceleration of the reference frame moving with respect to the inertial frame. The existence of the force of inertia is a property of any accelerated frame of reference.
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| Moment of inertia | | | The mechanics laws are invalid with respect to this frame of reference |