Frame of reference

A frame of reference is a coordinate system and clock convention used to assign positions and times to events. It specifies how measurements are made and can be stationary or moving relative to other frames.

Coordinate system: A mathematical scheme that assigns coordinates to points or events. A frame uses coordinates to identify where events occur.
Galilean transformation: The classical equations relating space and time coordinates in frames moving at constant relative velocity. It converts event measurements between inertial frames in Newtonian mechanics.
Observer (relativity): An idealized measuring system or person that records physical events. An observer may use a frame, but the observer and the coordinate convention are not identical.
Center-of-momentum frame: A frame in which a system's total momentum is zero. It simplifies collision and decay calculations by removing the system's net motion.
Galileo Galilei: An Italian scientist whose work helped establish mathematical physics and the principle of relativity for uniform motion. His ship argument illustrated why uniform motion cannot be detected by experiments confined to a frame.
Clock: A device or convention that assigns times to events. A frame needs a clock convention to assign event times.
Lorentz transformation: The equations relating spacetime coordinates between inertial frames in special relativity. It replaces Galilean transformations when the speed of light must remain invariant.
Reference point: A selected location used to describe position or displacement. A single point can anchor a measurement without specifying a full frame or clock.
Comoving coordinates: Coordinates that expand with the large-scale motion of the universe. They describe cosmic expansion while keeping galaxies approximately fixed in coordinate position.
Isaac Newton: An English mathematician and physicist who formulated classical mechanics and universal gravitation. His distinction between absolute and relative motion framed later disputes about reference systems.

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