Law Of Motion

There are three laws of motion and they were first compiled by Sir Isaac Newton in his work Philosophiæ Naturalis Principia Mathematica, first published on July 5, 1687^. Newton used these laws to explain and investigate the motion of many physical objects and systems. Newton showed that these laws of motion when combined with his law of universal gravitation, explained Kepler's laws of planetary motion.
Newton's laws are applied only to bodies (objects) which are considered or idealized as a particle, in the sense that the extent of the body is neglected in the evaluation of its motion, i.e.,the object is small when compared to the distances involved in the analysis, or the deformation and rotation of the body is of no importance in the analysis. Therefore, a planet is idealized as a particle for analysis of its orbital motion around a star.
Laws of motion are described as follows:

• First law: Every body remains in its state of rest or uniform motion (constant   velocity) unless it is compelled by an external unbalanced force to change that state. It means that in the absence of a non-zero or the net force the center of a mass of a body either remains at rest, or moves at a constant speed in a straight line.
• Second law: The rate of momentum of a body is directly proportional to the impressed force and takes place in the direction of the force. It means a body of mass m subject to a force F undergoes an acceleration a that has the same direction as the force and a magnitude that is directly proportional to the force and inversely proportional to the mass, i.e., F= ma. Alternatively, the total force applied on a body is same to that of the time derivative of the linear momentum of the body.
• Third law: To every action, there is equal and opposite reaction or the mutual forces of action and reaction between two bodies are equal, opposite and collinear. It means that whenever a first body exerts a force F on a second body, the second body exerts a force   -F on the first body. F and -F are equal in magnitude and opposite in direction. This law is usually referred to as action-reaction law with F called the "action" and -F the "reaction".