1.1. Newton's Laws of Motion (1): Inertia

Newton's first law describes the common property of matter, inertia.

 

$$ \sum F=0\Leftrightarrow \frac{d\vec{v}}{dt}=0 $$

1.1.1.1 Newton's First Law

 

Every particle maintains with no external influence its state of motion. In short, every static particle remains static and every moving particle remains moving.

 

To mathematically describe the motion of a particle, one needs a reference frame. It is obvious that an origin point and one or more axis is necessary to describe motion on the first or higher dimension. Reference frames where Newton's first law applies is called the inertial frame. An accelerating frame like a running bus is not an inertial frame.

 

 Because gravity propagates infinitely, an absolute inertial frame does not exist. Even the frames centering the surface of the Earth, the sun, or the Milky Way are accelerating. The fact that an absolute inertial frame does not exist leads the ingenious Albert Einstein to his famous theory of relativity.