Newtonian mechanics is the study of the causal relationship, in the natural world, between force, mass, and motion. Natural philosopher and 17th century scientist Isaac Newton developed a set of universal principles, elegant in their simplicity, to help explain and predict the motion of objects in the natural world, and the degree to which these objects change their relative motion by interacting with external forces. Newton embodied the interrelationship between the physical concepts of force, mass, and acceleration into his three laws of motion. The ability of these mechanics to accurately describe natural phenomenon under observation is derived from the application of these distinct laws of motion.
The first law of motion holds that a body in motion tends to remain in motion, and a body at rest tends to stay at rest. This principle explains the concept of inertia, namely, the application of force that is required to move a stationary object. Similarly, the deceleration of a body, otherwise moving at a constant speed, can only occur when an outside force acts on it. For example, a bullet fired from a rifle would continue its motion in a certain direction perpetually, were it not for the simultaneous forces of gravity and the resistance of the air in the atmosphere. These forces act together on the bullet to cause it to stop at a certain distance from the spot where it was initially fired.
Newton’s second law of motion is a mathematical or quantitative formula that describes the inherent nature of force. Newton postulated that the amount of force exerted is directly proportional to the mass of a body, times its acceleration, or f=ma. If two distinct bodies are moving at a constant acceleration, the object with the larger mass will produce the greater force. This principle can be illustrated by the example of an automobile and a locomotive that are traveling towards each other at an equal rate of speed. When the two objects collide, the force exerted on the automobile will be vastly greater due to the much greater mass of the locomotive.
The third law of motion can be summarized with the statement for every action, there is an equal and opposite reaction. In other words, the forces of two bodies acting upon each other are always equal and directly opposite. For instance, the force that a baseball exerts on a bat is equal and opposite to the force that the bat exerts on the baseball.
Newtonian mechanics provides analytical tools that an observer can use to accurately predict changes in a body’s motion resulting from external forces acting upon it. These principles are equally applicable to the movement of large celestial objects or the motion of a simple tennis ball. By using vectors to depict the magnitude and direction of a body in motion, as well as the manner in which external forces impact that body, an observer can accurately predict the net resultant velocity and direction of a body — which is the sum of all the external forces acting on that body at any given point in time.