The Scope of Physics

The scope of physics
The traditionally organized branches or fields of classical and

modern physics are delineated below.
Mechanics
Mechanics is generally taken to mean the study of the motion of
objects (or their lack of motion) under the action of given forces.
Classical mechanics is sometimes considered a branch of applied
mathematics. It consists of kinematics, the description of motion,
and dynamics, the study of the action of forces in producing either
motion or static equilibrium (the latter constituting the science
of statics). The 20th-century subjects of quantum mechanics,
crucial to treating the structure of matter, subatomic
particles, superfluidity, superconductivity, neutron stars, and other
major phenomena, and relativistic mechanics, important when
speeds approach that of light, are forms of mechanics that will be
discussed later in this section.
illustration of Robert Hooke's law of elasticity of materials

Illustration of Hooke's law of elasticity of materials, showing the stretching of a spring in
proportion to the applied force, from Robert Hooke's Lectures de Potentia Restitutiva (1678).
Photos.com/Jupiterimages
In classical mechanics the laws are initially formulated for point
particles in which the dimensions, shapes, and
other intrinsic properties of bodies are ignored. Thus in the first
approximation even objects as large as Earth and the Sun are
treated as pointlike—e.g., in calculating planetary orbital motion. In
rigid-body dynamics, the extension of bodies and their mass
distributions are considered as well, but they are imagined to be
incapable of deformation. The mechanics of deformable solids
is elasticity; hydrostatics and hydrodynamics treat, respectively,
fluids at rest and in motion.
The three laws of motion set forth by Isaac Newton form the

foundation of classical mechanics, together with the recognition
that forces are directed quantities (vectors) and combine
accordingly. The first law, also called the law of inertia, states that,
unless acted upon by an external force, an object at rest remains at
rest, or if in motion, it continues to move in a straight line with
constant speed. Uniform motion therefore does not require a cause.
Accordingly, mechanics concentrates not on motion as such but on
the change in the state of motion of an object that results from the
net force acting upon it. Newton’s second law equates the net force
on an object to the rate of change of its momentum, the latter being
the product of the mass of a body and its velocity. Newton’s third
law, that of action and reaction, states that when two particles
interact, the forces each exerts on the other are equal in magnitude
and opposite in direction. Taken together, these mechanical laws in
principle permit the determination of the future motions of a set of
particles, providing their state of motion is known at some instant,
as well as the forces that act between them and upon them from the
outside. From this deterministic character of the laws of classical
mechanics, profound (and probably incorrect) philosophical
conclusions have been drawn in the past and even applied to human
history.
Lying at the most basic level of physics, the laws of mechanics are
characterized by certain symmetry properties, as exemplified in the
aforementioned symmetry between action and reaction forces.
Other symmetries, such as the invariance (i.e., unchanging form) of
the laws under reflections and rotations carried out in space,
reversal of time, or transformation to a different part of space or to
a different epoch of time, are present both in classical mechanics
and in relativistic mechanics, and with certain restrictions, also in
quantum mechanics. The symmetry properties of the theory can be
shown to have as mathematical consequences basic principles
known as conservation laws, which assert the constancy in time of
the values of certain physical quantities under prescribed
conditions. The conserved quantities are the most important ones in
physics; included among them are mass and energy (in relativity
theory, mass and energy are equivalent and are conserved
together), momentum, angular momentum, and electric charge.

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The scope of physics

The traditionally organized branches or fields of classical and

illustration of Robert Hooke's law of elasticity of materials

The three laws of motion set forth by Isaac Newton form the

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