In common usage and
classical mechanics, a physical object or physical body (or simply an object or body) is a collection of
matter within a defined contiguous boundary in
three-dimensional space. The boundary must be defined and identified by the properties of the
material. The boundary may change over time. The boundary is usually the visible or tangible surface of the object. The matter in the object is constrained (to a greater or lesser degree) to move as one object. The boundary may move in space relative to other objects that it is not attached to (through translation and rotation). An object's boundary may also deform and change over time in other ways.
Also in common usage, an object is not constrained to consist of the same collection of
matter. Atoms or parts of an object may change over time. An object usually meant to be defined by the simplest representation of the boundary consistent with the observations. However the laws of physics only apply directly to objects that consist of the same collection of matter.
In
physics, an object is an
identifiable collection of
matter, which may be constrained by an identifiable boundary, and may move as a unit by
translation or rotation, in
3-dimensional space
Three-dimensional space (also: 3D space, 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called ''parameters'') are required to determine the position of an element (i.e., point). This is the informal ...
.
Each object has a unique identity, independent of any other properties. Two objects may be identical, in all properties except position, but still remain distinguishable. In most cases the boundaries of two objects may not overlap at any point in time. The property of identity allows objects to be counted.
Examples of
models
A model is an informative representation of an object, person or system. The term originally denoted the plans of a building in late 16th-century English, and derived via French and Italian ultimately from Latin ''modulus'', a measure.
Models c ...
of physical bodies include, but are not limited to a
particle, several
interacting
Interaction is action that occurs between two or more objects, with broad use in philosophy and the sciences. It may refer to:
Science
* Interaction hypothesis, a theory of second language acquisition
* Interaction (statistics)
* Interactions o ...
smaller bodies (
particulate or otherwise), and
continuous media.
The common conception of physical objects includes that they have
extension
Extension, extend or extended may refer to:
Mathematics
Logic or set theory
* Axiom of extensionality
* Extensible cardinal
* Extension (model theory)
* Extension (predicate logic), the set of tuples of values that satisfy the predicate
* E ...
in the
physical world, although there do exist
theories of
quantum physics
Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, qua ...
and
cosmology which arguably challenge this. In modern physics, "extension" is understood in terms of the
spacetime: roughly speaking, it means that for a given moment of
time the body has some location in the space (although not necessarily amounting to the abstraction of a
point
Point or points may refer to:
Places
* Point, Lewis, a peninsula in the Outer Hebrides, Scotland
* Point, Texas, a city in Rains County, Texas, United States
* Point, the NE tip and a ferry terminal of Lismore, Inner Hebrides, Scotland
* Point ...
in
space and time. A physical body as a whole is assumed to have such
quantitative
Quantitative may refer to:
* Quantitative research, scientific investigation of quantitative properties
* Quantitative analysis (disambiguation)
* Quantitative verse, a metrical system in poetry
* Statistics, also known as quantitative analysis ...
properties as
mass,
momentum
In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass an ...
,
electric charge, other
conserved quantities
In mathematics, a conserved quantity of a dynamical system is a function of the dependent variables, the value of which remains constant (mathematics), constant along each trajectory of the system.
Not all systems have conserved quantities, and c ...
, and possibly other quantities.
An object with known composition and described in an adequate physical theory is an example of
physical system
A physical system is a collection of physical objects.
In physics, it is a portion of the physical universe chosen for analysis. Everything outside the system is known as the environment. The environment is ignored except for its effects on the ...
.
In common usage
An object is known by the application of
senses. The properties of an object are inferred by learning and reasoning based on the information perceived. Abstractly, an object is a construction of our mind consistent with the information provided by our senses, using
Occam's razor
Occam's razor, Ockham's razor, or Ocham's razor ( la, novacula Occami), also known as the principle of parsimony or the law of parsimony ( la, lex parsimoniae), is the problem-solving principle that "entities should not be multiplied beyond neces ...
.
In common usage an object is the material inside the boundary of an object, in 3-dimensional space. The boundary of an object is a contiguous surface which may be used to determine what is inside, and what is outside an object. An object is a single piece of material, whose extent is determined by a description based on the properties of the material. An imaginary sphere of granite within a larger block of granite would not be considered an identifiable object, in common usage. A fossilized skull encased in a rock may be considered an object because it is possible to determine the extent of the skull based on the properties of the material.
For a
rigid body, the boundary of an object may change over time by
continuous
Continuity or continuous may refer to:
Mathematics
* Continuity (mathematics), the opposing concept to discreteness; common examples include
** Continuous probability distribution or random variable in probability and statistics
** Continuous ...
translation and
rotation
Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...
. For a
deformable body the boundary may also be continuously
deformed over time in other ways.
An object has an
identity
Identity may refer to:
* Identity document
* Identity (philosophy)
* Identity (social science)
* Identity (mathematics)
Arts and entertainment Film and television
* ''Identity'' (1987 film), an Iranian film
* ''Identity'' (2003 film), ...
. In general two objects with identical properties, other than position at an instance in time, may be distinguished as two objects and may not occupy the same space at the same time (excluding component objects). An object's identity may be tracked using the continuity of the change in its boundary over time. The identity of objects allows objects to be arranged in
sets and
counted.
The material in an object may change over time. For example, a rock may wear away or have pieces broken off it. The object will be regarded as the same object after the addition or removal of material, if the system may be more simply described with the continued existence of the object, than in any other way. The addition or removal of material may discontinuously change the boundary of the object. The continuation of the object's identity is then based on the description of the system by continued identity being simpler than without continued identity.
For example, a particular car might have all its wheels changed, and still be regarded as the same car.
The identity of an object may not split. If an object is broken into two pieces at most one of the pieces has the same identity. An object's identity may also be destroyed if the simplest description of the system at a point in time changes from identifying the object to not identifying it. Also an object's identity is created at the first point in time that the simplest model of the system consistent with perception identifies it.
An object may be composed of components. A component is an object completely within the boundary of a containing object.
Classical mechanics
In
classical mechanics a physical body is collection of matter having properties including
mass,
velocity,
momentum
In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass an ...
and
energy. The matter exists in a volume of
three-dimensional space. This space is its
extension
Extension, extend or extended may refer to:
Mathematics
Logic or set theory
* Axiom of extensionality
* Extensible cardinal
* Extension (model theory)
* Extension (predicate logic), the set of tuples of values that satisfy the predicate
* E ...
.
Under
Newtonian gravity
Newton's law of universal gravitation is usually stated as that every particle attracts every other particle in the universe with a force that is proportional to the product of their masses and inversely proportional to the square of the distanc ...
the gravitational field further away than the furthest extent of an object is determined only by the mass and the position of the center of mass.
Interactions between objects are partly described by orientation and external shape.
In
continuum mechanics
Continuum mechanics is a branch of mechanics that deals with the mechanical behavior of materials modeled as a continuous mass rather than as discrete particles. The French mathematician Augustin-Louis Cauchy was the first to formulate such m ...
an object may be described as a collection of sub objects, down to an infinitesimal division, which interact with each other by forces that may be described internally by
pressure and
mechanical stress.
Quantum mechanics
In
quantum mechanics an object is a particle or collection of particles. Until measured, a particle does not have a physical position. A particle is defined by a
probability distribution
In probability theory and statistics, a probability distribution is the mathematical function that gives the probabilities of occurrence of different possible outcomes for an experiment. It is a mathematical description of a random phenomenon i ...
of finding the particle at a particular position. There is a
limit to the accuracy with which the position and velocity may be measured. A particle or collection of particles is described by a
quantum state.
These ideas vary from the common usage understanding of what an object is.
String theory
In
particle physics, there is a debate as to whether some
elementary particles are not bodies, but are
points
Point or points may refer to:
Places
* Point, Lewis, a peninsula in the Outer Hebrides, Scotland
* Point, Texas, a city in Rains County, Texas, United States
* Point, the NE tip and a ferry terminal of Lismore, Inner Hebrides, Scotland
* Point ...
without
extension
Extension, extend or extended may refer to:
Mathematics
Logic or set theory
* Axiom of extensionality
* Extensible cardinal
* Extension (model theory)
* Extension (predicate logic), the set of tuples of values that satisfy the predicate
* E ...
in
physical space within
spacetime, or are always extended in at least one dimension of space as in
string theory
In physics, string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings. String theory describes how these strings propagate through space and interac ...
or
M theory
M-theory is a theory in physics that unifies all consistent versions of superstring theory. Edward Witten first conjectured the existence of such a theory at a string theory conference at the University of Southern California in 1995. Witten's ...
.
In psychology
In some branches of
psychology, depending on
school of thought, a physical object has
physical properties
A physical property is any property that is measurable, whose value describes a state of a physical system. The changes in the physical properties of a system can be used to describe its changes between momentary states. Physical properties are o ...
, as compared to
mental objects The mental world is an ontological category in metaphysics, populated by nonmaterial mental objects, without physical extension (though possibly with mental extension as in a visual field, or possibly not, as in an olfactory field) contrasted with ...
. In (
reductionistic)
behaviorism
Behaviorism is a systematic approach to understanding the behavior of humans and animals. It assumes that behavior is either a reflex evoked by the pairing of certain antecedent (behavioral psychology), antecedent stimuli in the environment, o ...
, objects and their properties are the (only)
meaningful
Semantics (from grc, σημαντικός ''sēmantikós'', "significant") is the study of reference, meaning, or truth. The term can be used to refer to subfields of several distinct disciplines, including philosophy, linguistics and compu ...
objects of study. While in the modern day behavioral psychotherapy it is still only the means for goal oriented behavior modifications, in
Body Psychotherapy it is not a means only anymore, but its felt sense is a goal of its own. In
cognitive psychology
Cognitive psychology is the scientific study of mental processes such as attention, language use, memory, perception, problem solving, creativity, and reasoning.
Cognitive psychology originated in the 1960s in a break from behaviorism, which ...
, physical bodies as they occur in
biology are studied in order to understand the
mind
The mind is the set of faculties responsible for all mental phenomena. Often the term is also identified with the phenomena themselves. These faculties include thought, imagination, memory, will, and sensation. They are responsible for various m ...
, which may not be a physical body, as in
functionalist schools of thought.
In philosophy
A physical body is an enduring object that exists throughout a particular
trajectory of
space and orientation over a particular duration of
time, and which is located in the
world of
physical space (i.e., as studied by
physics). This contrasts with
abstract object
In metaphysics, the distinction between abstract and concrete refers to a divide between two types of entities. Many philosophers hold that this difference has fundamental metaphysical significance. Examples of concrete objects include plants, hum ...
s such as
mathematical objects which do not exist at any particular time or place.
Examples are a
cloud, a
human body
The human body is the structure of a Human, human being. It is composed of many different types of Cell (biology), cells that together create Tissue (biology), tissues and subsequently organ systems. They ensure homeostasis and the life, viabi ...
, a
banana
A banana is an elongated, edible fruit – botanically a berry – produced by several kinds of large herbaceous flowering plants in the genus ''Musa''. In some countries, bananas used for cooking may be called "plantains", distinguis ...
, a billiard ball, a table, or a
proton
A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
. This is contrasted with abstract objects such as
mental object The mental world is an ontological category in metaphysics, populated by nonmaterial mental objects, without physical extension (though possibly with mental extension as in a visual field, or possibly not, as in an olfactory field) contrasted with t ...
s, which exist in the
mental world, and
mathematical objects. Other examples that are ''not''
physical bodies
In common usage and classical mechanics, a physical object or physical body (or simply an object or body) is a collection of matter within a defined contiguous boundary in three-dimensional space. The boundary must be defined and identified by t ...
are
emotions, the concept of "
justice", a feeling of hatred, or the
number "3". In some philosophies, like the
idealism of
George Berkeley
George Berkeley (; 12 March 168514 January 1753) – known as Bishop Berkeley (Bishop of Cloyne of the Anglican Church of Ireland) – was an Anglo-Irish philosopher whose primary achievement was the advancement of a theory he called "immate ...
, a physical body ''is'' a
mental object The mental world is an ontological category in metaphysics, populated by nonmaterial mental objects, without physical extension (though possibly with mental extension as in a visual field, or possibly not, as in an olfactory field) contrasted with t ...
, but still has extension in the space of a visual field.
See also
*
Abstract object theory
*
Deformable body
*
Human body
The human body is the structure of a Human, human being. It is composed of many different types of Cell (biology), cells that together create Tissue (biology), tissues and subsequently organ systems. They ensure homeostasis and the life, viabi ...
*
Non-physical entity
*
Physical model
A model is an informative representation of an object, person or system. The term originally denoted the Plan_(drawing), plans of a building in late 16th-century English, and derived via French and Italian ultimately from Latin ''modulus'', a mea ...
*
Rigid body
*
Ship of Theseus, a thought experiment about an object's identity over time
External links
*
{{Authority control
Concepts in metaphysics
Concepts in physics
Mechanics
Ontology