Newtonian mechanics, or classical mechanics, is the branch of physics that studies the motion of bodies in accordance with the general principles first set down by Sir Isaac Newton in his Philosophiæ Naturalis Principia Mathematica (1687). These principles are now collectively known as Newton’s laws of motion. Classical mechanics is generally concerned with the motion of macroscopic objects, from baseballs to stars and galaxies. microscopic realm of atomic and subatomic particles.
In its simplest form, classical mechanics postulates that there is no action at a distance; instead, all interactions between objects take place through some kind of contact force. This force may be an actual physical push or pull (as when one object collides with another), or it may be an invisible electromagnetic field (as when one charged particle exerts a force on another). In either case, the net force acting on any object is equal to the sum of all the individual forces acting on that object.
The second law of motion states that the acceleration produced by a given force is directly proportional to the magnitude of the force and inversely proportional to the mass of the object being accelerated. The acceleration produced by a given force will always be in the same direction as that force. This law can be expressed mathematically as follows:
F = ma ,where F isthe appliedforce(measuredinnewtons), misthemassofbodybeingaccelerated(kilograms),andais thenewtonspersecond persecond(acceleration).
The third law states that for every action there is an equal and opposite reaction. This means that if one object exerts a force on another object, then the second object will exert an equal and opposite force back on the first. The two forces will be exerted on different objects; they will not cancel each other out because they are not acting on the same body. For example, when you walk forward, your legs push backward against the ground; at the same time, though, the ground pushes your legs forward so that you can keep moving. It is important to note that although action-reaction pairs always involve two different objects, they do not necessarily involve two different types of forces. For example, if you jump up into air (an unbalanced interaction), then gravity immediately begins pulling you back down toward Earth (a balanced interaction). Action-reaction pairs involving gravitation are discussed later in this article.
The fourth law—sometimes called Newton’s law of universal gravitation—states that every particle inthe universeattractseveryotherparticlewithagravitationalforcethatisdirectlyproportionaltotheproductoftheirtwo massesandinverselyproportionaltothe squareofthedistancebetweenthem:
F=Gm1m2/d^2 ,whereFisthegravitationalforceexertedbyoneobjectonanother(innewtons),Gistheuniversalgravitationalconstant(6 .674×10−11 N·m^2 /kg^2 ),mis themassofthefirstobject(inkilograms),misthemassofthesecondobject(inkilograms),anddis theseparationbetweenthetwobodies(ind meters).Thislawcanbeusedtocalculatethegravitationalforces betweensolarsystembodiesaswellasthelargescale structureoftheuniverseitself .
Newtons laws are valid only under certain conditions—namely, those where accelerations are small enough and speeds are low enough suchthat relativistic effects can safely be ignored . At high speeds or strong gravitational fields , Newtonian mechanics breaksdown and mustbe replacedwith Einstein ‘sg theoryofrelativityorwith quantummechanics ,respectively .