Motion is never absolute; it depends entirely on your perspective
In short: Is a passenger sitting still if the train is moving? In physics, the answer depends on your frame of reference. To describe how an object moves, you must first choose a system of measurement to define its position relative to everything else.
In dynamics, a reference frame acts as a symbolic system of graduated lines attached to a body. These lines allow us to describe the position of specific points relative to that body. To find a location on Earth, we employ a coordinate system using latitude degrees from the Equator and longitude degrees from the great circle connecting the poles and Greenwich, England.
These systems are often called coordinate systems, featuring axes that radiate from a central point called the origin. Depending on the complexity of the movement, scientists use different methods to map it. For motion on a flat plane, one might use rectangular coordinates—measuring distances from two perpendicular lines—or polar coordinates, which use a fixed origin, a distance, and an angle. For three-dimensional motion, scientists may employ spherical coordinates, utilizing a distance and two angles equivalent to longitude and latitude.
The way we perceive motion is deeply tied to these frames. A key concept is the position vector, which is a line drawn from the origin to a point. As the point moves, this vector changes in both direction and magnitude, and these changes define the object's velocity. This relationship is central to understanding how objects interact in space.
Interestingly, Newton’s laws of motion are strictly valid only within an inertial reference frame—a coordinate system that remains at rest relative to the 'fixed' stars. However, according to the principle of Newtonian or Galilean relativity, these laws also hold true in any rigid coordinate system that moves with a constant velocity and without rotation relative to that inertial frame.
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Sources
- Summary by Intellect · source: Frames of reference | Movement and forces | Middle school physics | Khan Academy
- Also: Reference frame | Definition, Types & Examples | Britannica
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