Notes on Relative Motion and Reference Frames

All motion is relative; objects are constantly in motion at various speeds (e.g., Earth's rotation, orbit, galactic movement).
Everyday motion is typically discussed relative to the Earth's surface, treating it as stationary despite its actual speed.

An inertial reference frame is a chosen object, person, or location considered completely still, against which the motion of other objects is assessed.
Perceived motion changes based on the chosen inertial reference frame; for example, a person in a constantly moving car feels stationary while surroundings appear to move.
It's equally valid to consider a moving object (like a car) as an inertial reference frame as it is to consider the Earth.

Galileo was the first to rigorously describe relative motion, demonstrating through experiments on a moving ship that objects fall straight down regardless of constant ship velocity.
He proved that velocity measurements are dependent on the adopted reference frame.
This work, known as Galilean relativity, established fundamental concepts that were later built upon by Einstein's special relativity.
For many physics studies on Earth, despite its motion, it serves as a good approximation of an inertial reference frame.