Describing Motion: Coordinates, Reference Frames and Relativity of Motion
How physicists say exactly where a body is and how it moves
Big Idea: Motion is always relative. A body is never simply "moving" or "at rest" — it moves or rests only with respect to some chosen reference frame. To turn the vague statement "the bus is moving" into real physics we pick a body of reference, attach a coordinate system and a clock to it, and then track how the body's coordinate changes with time.
By the end you will be able to:
- decide when a body may be treated as a material point,
- name the three parts of a reference frame,
- find the coordinate, displacement and path of a moving body,
- explain why velocity and trajectory depend on the observer,
- use the law of addition of velocities,
- say what an inertial reference frame is.