Inertiell

Inertiell is an adjective used in physics and related disciplines to describe properties, systems, or frames of reference that pertain to inertia. The term is commonly encountered in German-language literature, where the noun inertia is “Trägheit” and the adjective “inertiell” denotes things associated with inertia. In English, the equivalent term is inertial.

Inertia is the resistance of any physical object to a change in its state of motion or rest. This property is quantified by mass. An inertial frame of reference is one in which Newton's first law holds: a body not acted upon by external forces moves with constant velocity.

Non-inertial frames require fictitious forces; rotating frames yield centrifugal and Coriolis forces. In special relativity, inertial frames move with uniform velocity relative to each other; in general relativity, local inertial frames exist in curved spacetime and are related to free fall.

Inertiell concepts influence instrumentation and analysis, such as inertial navigation systems that use accelerometers and gyroscopes to track motion relative to an inertial frame, often without external references. In materials science and engineering, inertial properties describe how matter resists changes in motion due to mass and internal structure, with implications for design and dynamics in mechanical systems.