Inertials

Inertials, in physics, refer to the concept of inertial frames of reference and related notions such as inertia. An inertial frame is a frame of reference in which a body not subjected to external forces moves with constant velocity, and in which Newton's laws hold in their simplest form.

Within inertial frames, the equations of motion are the same as in any other, and the results are invariant under Galilean transformations. This invariance under uniform motion means that no experiment confined to a closed laboratory can distinguish between inertial frames, in principle.

Non-inertial frames, by contrast, are accelerating or rotating relative to an inertial frame and exhibit fictitious forces such as the Coriolis and centrifugal forces. These forces are not real interactions but arise from the frame's acceleration.

Relativity adds nuance: in special relativity, inertial frames move at constant velocity relative to one another and are related by Lorentz transformations. In general relativity, there is no global inertial frame in curved spacetime; locally, free-falling frames are approximately inertial.

Inertia is the resistance to changes in motion and is quantified by inertial mass, a quantity that determines how much a body accelerates under a given force. The equivalence principle ties inertial and gravitational mass together, a cornerstone of general relativity.

Practically, inertial measurement is implemented with inertial measurement units (IMUs), which combine accelerometers and gyroscopes to measure linear acceleration and angular motion. These devices underpin inertial navigation systems in aerospace, ships, vehicles, and consumer electronics.