Mechanics is the branch of physics that studies the motion of bodies. The Greek philosophers were among the first to propose abstract ideas about what motion means. The experimental scientific method was first introduced by the Islamic scientists during the middle ages. However, it was not until the works of Galileo Galilei and Isaac Newton that the foundations of what it is now known as classical mechanics were established.
Mechanics can be broadly divided into classical mechanics, relativistic mechanics and quantum mechanics.
Classical mechanics: Describes the motion of macroscopic bodies, which travel at a velocity much less than the speed of light. There are several formulations of classical mechanics. The most widely known is Newtonian mechanics, as was established in Isaac Newton’s book “Principia Mathematica”. It is based on the fact that bodies accelerate whenever they are under the influence of a force. Classical mechanics can also be studied from the point of view of conservation of energy, or with the more abstract formulations of Lagrangian Dynamics and Hamiltonean Dynamics. According to classical mechanics, gravity is an “action at a distance” force between two objects, that diminish the more separated the objects are.
Relativistic Mechanics: Describe the motion of bodies which travel at speeds comparable, but less, than the speed of light. Classical mechanics is an approximation of relativistic mechanics when the object travels at a much lower speed. Its main ideas is that all the laws of physics are the same no matter how fast the observer is moving, as long as it is moving at a constant speed, that the speed of light is the same for all observers, again, independently of how fast they are moving, and that gravity is not a force, but a curvature of space time caused by a massive object and “felt” by another massive object. Some authors consider relativistic mechanics as part of classical mechanics, and only make distinction between classical and quantum mechanics.
Quantum mechanics: Study the motion of the bodies at very low energy scales. (Or at a microscopic level). It was developed in the first half of the XX century by physicists like Heisenberg, Planck, Einstein, Schrödinger and others. According to quantum mechanics, all objects behave at the same time as particles and as waves. For example, we could view light as an electromagnetic wave, or as a stream of particles called photons. Similarly, all ordinary objects have wave like properties, although almost unnoticeable for macroscopic objects. Also, instead of the certain predictions of classical mechanics, quantum mechanics can only make probabilistic statements, both of the current state and of the future of the object in study.