In common usage and

classical mechanics Classical mechanics is a physical theory describing the motion of macroscopic objects, from projectiles to parts of machinery, and astronomical objects, such as spacecraft, planets, stars, and galaxies. For objects governed by classi ...

, a physical object or physical body (or simply an object or body) is a collection of

matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic part ...

within a defined contiguous boundary in

three-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 informa ...

. The boundary must be defined and identified by the properties of the

material Material is a substance or mixture of substances that constitutes an object. Materials can be pure or impure, living or non-living matter. Materials can be classified on the basis of their physical and chemical properties, or on their geolo ...

. 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

. 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 Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...

, an object is an

identifiable In statistics, identifiability is a property which a model must satisfy for precise inference to be possible. A model is identifiable if it is theoretically possible to learn the true values of this model's underlying parameters after obtaining an ...

collection of

, which may be constrained by an identifiable boundary, and may move as a unit by

translation Translation is the communication of the meaning of a source-language text by means of an equivalent target-language text. The English language draws a terminological distinction (which does not exist in every language) between ''transla ...

or rotation, in 3-dimensional space. 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 of physical bodies include, but are not limited to a

particle In the physical sciences, a particle (or corpuscule in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass. They vary greatly in size or quantity, from ...

, 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 Particulates – also known as atmospheric aerosol particles, atmospheric particulate matter, particulate matter (PM) or suspended particulate matter (SPM) – are microscopic particles of solid or liquid matter suspended in the air. The te ...

or otherwise), and

continuous media Continuum mechanics is a branch of mechanics that deals with the mechanical behavior of materials modeled as a continuous mass rather than as point particle, discrete particles. The French mathematician Augustin-Louis Cauchy was the first to fo ...

. The common conception of physical objects includes that they have extension in the

physical world The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. Acco ...

, although there do exist

theories A theory is a rational type of abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated with such processes as observational study or research. Theories may be ...

of quantum physics and

cosmology Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe. The term ''cosmology'' was first used in English in 1656 in Thomas Blount's ''Glossographia'', and in 1731 taken up in Latin by German philosopher ...

which arguably challenge this. In modern physics, "extension" is understood in terms of the

spacetime In physics, spacetime is a mathematical model that combines the three dimensions of space and one dimension of time into a single four-dimensional manifold. Spacetime diagrams can be used to visualize relativistic effects, such as why differ ...

: roughly speaking, it means that for a given moment of

time Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future. It is a component quantity of various measurements used to sequence events, ...

the body has some location in the space (although not necessarily amounting to the abstraction of a point in

space and time Space and Time or Time and Space, or ''variation'', may refer to: * '' Space and time'' or ''time and space'' or ''spacetime'', any mathematical model that combines space and time into a single interwoven continuum * Philosophy of space and time S ...

. 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 Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different eleme ...

, momentum,

electric charge Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respe ...

, other conserved quantities, and possibly other quantities. An object with known composition and described in an adequate physical theory is an example of physical system.

In common usage

An object is known by the application of

sense A sense is a biological system used by an organism for sensation, the process of gathering information about the world through the detection of stimuli. (For example, in the human body, the brain which is part of the central nervous system re ...

s. 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. 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 In physics, a rigid body (also known as a rigid object) is a solid body in which deformation is zero or so small it can be neglected. The distance between any two given points on a rigid body remains constant in time regardless of external fo ...

, 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 ...

and rotation. For a

deformable body In physics and materials science, plasticity, also known as plastic deformation, is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces. For example, a solid piece ...

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

a physical body is collection of matter having properties including

velocity Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity i ...

, momentum and

energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of hea ...

. The matter exists in a volume of

. This space is its extension. 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 distan ...

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 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 Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

and

mechanical stress In continuum mechanics, stress is a physical quantity. It is a quantity that describes the magnitude of forces that cause deformation. Stress is defined as ''force per unit area''. When an object is pulled apart by a force it will cause elonga ...

Quantum mechanics

quantum mechanics 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 chemistr ...

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 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 In quantum physics, a quantum state is a mathematical entity that provides a probability distribution for the outcomes of each possible measurement on a system. Knowledge of the quantum state together with the rules for the system's evolution i ...

. These ideas vary from the common usage understanding of what an object is.

String theory

particle physics Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) an ...

, there is a debate as to whether some

elementary particle In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. Particles currently thought to be elementary include electrons, the fundamental fermions ( quarks, leptons, a ...

s are not bodies, but are points without extension in

physical space Space is the boundless three-dimensional extent in which objects and events have relative position and direction. In classical physics, physical space is often conceived in three linear dimensions, although modern physicists usually conside ...

within

, or are always extended in at least one dimension of space as in string theory or M theory.

In psychology

In some branches of

psychology Psychology is the scientific study of mind and behavior. Psychology includes the study of conscious and unconscious phenomena, including feelings and thoughts. It is an academic discipline of immense scope, crossing the boundaries between ...

, depending on

school of thought A school of thought, or intellectual tradition, is the perspective of a group of people who share common characteristics of opinion or outlook of a philosophy, discipline, belief, social movement, economics, cultural movement, or art movement. ...

, a physical object has physical properties, as compared to mental objects. In (

reductionistic Reductionism is any of several related philosophical ideas regarding the associations between phenomena which can be described in terms of other simpler or more fundamental phenomena. It is also described as an intellectual and philosophical po ...

) behaviorism, objects and their properties are the (only) meaningful objects of study. While in the modern day behavioral psychotherapy it is still only the means for goal oriented behavior modifications, in

Body Psychotherapy Body psychotherapy, also called body-oriented psychotherapy, is an approach to psychotherapy which applies basic principles of somatic psychology. It originated in the work of Pierre Janet, Sigmund Freud and particularly Wilhelm Reich who develope ...

it is not a means only anymore, but its felt sense is a goal of its own. In cognitive psychology, physical bodies as they occur in

biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary i ...

are studied in order to understand the mind, 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 A trajectory or flight path is the path that an object with mass in motion follows through space as a function of time. In classical mechanics, a trajectory is defined by Hamiltonian mechanics via canonical coordinates; hence, a complete tra ...

space Space is the boundless three-dimensional extent in which objects and events have relative position and direction. In classical physics, physical space is often conceived in three linear dimensions, although modern physicists usually cons ...

and orientation over a particular duration of

, and which is located in the

world In its most general sense, the term "world" refers to the totality of entities, to the whole of reality or to everything that is. The nature of the world has been conceptualized differently in different fields. Some conceptions see the worl ...

(i.e., as studied by

). This contrasts with abstract objects such as

mathematical object A mathematical object is an abstract concept arising in mathematics. In the usual language of mathematics, an ''object'' is anything that has been (or could be) formally defined, and with which one may do deductive reasoning and mathematical p ...

s which do not exist at any particular time or place. Examples are a

cloud In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may ...

, a human body, a banana, a billiard ball, a table, or a proton. This is contrasted with abstract objects such as mental objects, which exist in the

mental world 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 ...

, and

s. 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 Emotions are mental states brought on by neurophysiological changes, variously associated with thoughts, feelings, behavioral responses, and a degree of pleasure or displeasure. There is currently no scientific consensus on a definition. ...

, the concept of "

justice Justice, in its broadest sense, is the principle that people receive that which they deserve, with the interpretation of what then constitutes "deserving" being impacted upon by numerous fields, with many differing viewpoints and perspective ...

", a feeling of hatred, or the

number A number is a mathematical object used to count, measure, and label. The original examples are the natural numbers 1, 2, 3, 4, and so forth. Numbers can be represented in language with number words. More universally, individual numbers c ...

"3". In some philosophies, like the

idealism In philosophy, the term idealism identifies and describes metaphysical perspectives which assert that reality is indistinguishable and inseparable from perception and understanding; that reality is a mental construct closely connected t ...

of George Berkeley, a physical body ''is'' a mental object, but still has extension in the space of a visual field.

External links

* {{Authority control Concepts in metaphysics Concepts in physics Mechanics Ontology