In physics, mechanics is concerned with the motion of objects when acted upon by forces. For historical reasons, classical mechanics is usually limited to descriptions based upon Newtonian physics (i.e., Newton's laws of motion). In contrast, quantum mechanics utilizes quantum theory to describe the behavior of subatomic particles. Classical mechanics also differs from statistical or relativistic mechanics. Statistical mechanics is concerned with large numbers of objects. Relativistic mechanics studies objects moving at velocities close to the speed of light, or whose kinetic energies are on the scale of the product of their masses and the square of the velocity of light (mc2 ). The subdivisions of mechanics include kinematics, which studies the movement of objects without explicit consideration of the forces responsible for the motion, and dynamics, which is concerned with the motion of moving objects and the forces and physical properties that affect them (e.g., mass, momentum, and energy). The special case in which any number of forces balance each other out and cause a body to remain at unaccelerated velocity (as in equilibrium), is covered by statistics.
The foundations of modern mechanics were laid in the early seventeeth century by Italian astronomer and physicist Galileo who studied the motion of simple objects. Galileo realized that, when not subjected to external forces, objects in motion continued to move and that the application of a force was required to change their motion. Besides this principle of inertia, Galileo discovered that free-falling objects move with constant acceleration. These concepts led English physicist Sir Isaac Newton to formulate his laws of motion, which represent the basic principles of mechanics, and later develop his law of universal gravitation.
In mechanics, motion is described using the concepts of velocity and acceleration. In turn, forces causing motion are studied by observing the resultant motion of bodies. The first law states that an object will remain at constant velocity (move at the same speed and in the same direction) unless acted upon by an external force. The law also implies the concepts of inertia and frames of reference. The second law defines force quantitatively as the product of a body's mass and acceleration (F = ma) and is a formulation of the fundamental cause of motion that applies to a wide range of physical phenomena. The third law, which states that there are no isolated forces and that forces come in pairs, implies a profound physical principle important to the understanding of forces, such as friction and thrust. Mechanics is also concerned with the study of the mechanical properties of a system interacting with its environment. These properties, such as energy, momentum, and angular momentum, are said to be conserved and the conservation laws that describe their behavior represent the most fundamental principles of mechanics. Mechanics is also concerned with thermodynamics quantities such as work (the application of force in the direction of the displacement of a body), energy (the capacity to perform work), and power (the rate at which work is performed).
Mechanics has a significant impact on day-to-day life, simply because almost everything moves in the physical world. Applications of mechanics include the study of rectilinear motion; displacement of motor-driven machines; motion under the influence of gravity (including projectile motion and ballistics); periodic motion; clock and time-keeping devices; circular motion; rotational motion; and motions of bodies following collisions.
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