Inertialsystemen

Inertialsystemen, or inertial reference frames, are coordinate systems in which Newton's laws take their simplest form. In an inertial frame, a free particle—one not subjected to any net external force—moves with constant velocity. Equivalently, the acceleration of a body equals the net external force divided by its mass, without the need for fictitious forces.

Classically, inertial frames are related by Galilean transformations, which include shifts in position, uniform motion, and rotations. In these transformations, the form of physical laws remains unchanged, time is absolute, and there is no preferred state of rest.

Real systems are never perfectly inertial. The Earth is approximately inertial for short experiments, but its rotation introduces fictitious forces such as the Coriolis and centrifugal effects. In space, inertial frames can be defined by freely moving reference markers and gyroscopes; inertial navigation systems use accelerometers and gyroscopes to maintain an effective inertial frame for tracking motion.

Relativistic theories extend the concept. In Special Relativity, all inertial frames moving at constant velocity relative to one another are related by Lorentz transformations, and the laws of physics are the same in each frame. In General Relativity, gravity is described as spacetime curvature, and only local inertial frames exist within small regions of spacetime—the equivalence principle allows one to treat small patches as inertial even in a gravitational field, though a single global inertial frame may not exist.