Inertial frame of reference

A reference frame in which force-free bodies move at constant velocity and Newton’s laws hold without fictitious forces.

Newton's first law: A body remains at rest or moves at constant velocity unless acted on by a net force. It identifies inertial frames by the motion of force-free bodies.
Reference frame: A coordinate system, together with a clock, used to describe positions and motion. An inertial frame is a particular kind of reference frame.
Non-inertial frame of reference: A reference frame that accelerates or rotates relative to an inertial frame. Newton’s laws require fictitious forces when written in this kind of frame.
Laboratory frame: A coordinate frame fixed to a laboratory, often treated as approximately inertial over limited times and distances. Experiments commonly use it as a practical approximation to an inertial frame.
Newton's second law: The net force on a body equals its mass times its acceleration in classical mechanics. Its standard form applies directly in an inertial frame.
Inertia: The resistance of matter to changes in its state of motion. Inertial motion is the defining behavior used to identify these frames.
Rotating reference frame: A reference frame whose axes rotate relative to an inertial frame. Rotation produces apparent forces absent from inertial-frame descriptions.
Center-of-momentum frame: A frame in which the total momentum of a system is zero. It is a useful inertial frame for analyzing collisions and particle interactions.
Galilean transformation: A coordinate transformation relating positions and times in classical inertial frames moving at constant relative velocity. It connects the descriptions of motion in different classical inertial frames.
Inertial mass: A measure of how strongly an object resists acceleration under an applied force. It appears in Newton’s law of motion in an inertial frame.

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