inertial

Inertial is an adjective used in physics to describe properties related to inertia, the resistance of any physical body to changes in its state of motion. Formally, a body at rest tends to stay at rest and a body in motion tends to move with constant velocity unless acted on by a net external force.

In physics, an inertial reference frame is a frame of reference in which Newton's laws hold without the need to invoke fictitious forces. In Newtonian mechanics, inertial frames are related by Galilean transformations; in special relativity, they are related by Lorentz transformations. A frame that accelerates or rotating is non-inertial because observers in it must introduce fictitious forces to apply Newton’s laws.

Inertia is the property that gives mass its resistance to acceleration. This resistance is quantified by inertial mass through the relation F = ma. The equivalence principle states that inertial mass and gravitational mass are experimentally indistinguishable, a foundational idea behind general relativity, where locally inertial frames exist in freely falling regions of spacetime.

Practical uses of the concept appear in inertial navigation systems (INS), which estimate position and velocity by integrating measurements from accelerometers and gyroscopes. INS devices are common in aircraft, ships, submarines, and spacecraft, and they demonstrate the operational side of inertia by continuing motion absent external references. Overall, the inertial concept underpins foundational mechanics and a range of technologies that rely on the predictable resistance of matter to changes in motion.