Newton's law of motion.

Sol.: The entire system of Dynamics is based on the three laws of motion that are basis assumptions, and were initially formulated by Newton.

First Law

A particle remains at rest or continues to move in a straight line with a constant speed. If the resultant force that is acting on it is Zero.

It is called as law of inertia, and consists of the following two parts: 

1) A body at rest has tendency to remain at rest. It is called as inertia of rest.

2) A body in motion has tendency to preserve its motion. It is called as inertia of motion.