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.
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.