● particle(질점) -  is a body whose size does not influence its response to the forces acting on it.

-  Rotational motion of the body can be ignored.

-  Simplest mechanical system is a single particle.

-  The body may be modeled as a point.

● rigid body(강체) -  is a body that does not deform under the action of forces.

1.2 Newton’s law of motion

● First law(관성의 법칙) – In the absence of applied forces, a particle originally at rest or moving with constant speed in a straight line will remain at rest or continue to move with constant speed in a straight line - > “special case of the second law”

정지=>정지, 운동=>운동

● Second law(가속도의 법칙) = If a particle is subjected to a force, the particle will be accelerated in the direction of the force, and the magnitude of the acceleration will be proportional to the magnitude fo the force.

F = ma,  where, F: force, m: mass, a: acceleration 

● Third law(작용과 반작용의 법칙) -  Every force (or action) is accompanied by an equal and opposite force (reaction).

- > This law can be extended to a system of two or more particles acted on by a system of forces. 

- > But second law assumes that a single force acts on a single particle