An inertial frame is a theoretical reference frame (coordinate system)
in which under the influence of no forces
an object will remain at rest or in uniform motion.
The concept is a direct implication of Newton's 1st law.
Accelerating (or rotating) frames are not inertial frames.
Obviously, given the universal force of gravity,
no body can actually be under the influence of no forces.
However the concept of inertial frames is
- still useful since, at
least theoretically, the effect of gravity can be very small.
Thus physical insight can be obtained
(e.g. Einstein's Special Theory of Relativity).
- valid under Einstein's General Theory of Relativity:
a frame in free-fall under the influence of gravity is
an inertial frame.
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