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In [[Physics:Physics|physics]], a '''physical body''' or '''physical object''' (or simply a '''body''' or '''object''') is an [[Philosophy:Identity|identifiable]] collection of [[Physics:Matter|matter]], which may be constrained by an identifiable boundary, and may move as a unit by translation or rotation, in [[Three-dimensional space|3-dimensional space]].

In common usage, an object is a collection of matter within a defined contiguous boundary in 3-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 [[Physics:Matter|matter]].  Atoms or parts of an object may change over time.  An object is 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.

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 [[Physics:Particle|particle]], several [[Interaction|interacting]] smaller bodies (particles or other), and continuous media.

The common conception of physical objects includes that they have [[Philosophy:Extension (metaphysics)|extension]] in the [[Astronomy:Universe|physical world]], although there do exist theories of quantum physics and [[Astronomy:Cosmology|cosmology]] which may challenge{{how|date=June 2013}} this. In modern physics, "extension" is understood in terms of the [[Physics:Spacetime|spacetime]]: roughly speaking, it means that for a given moment of [[Time|time]] the body has some location in the space, although not necessarily a [[Point (geometry)|point]]. A physical body as a whole is assumed to have such [[Physics:Physical quantity|quantitative]] properties as [[Physics:Mass|mass]], [[Physics:Momentum|momentum]], [[Physics:Electric charge|electric charge]], other [[Physics:Conservation law|conserving quantities]], and possibly other quantities.

A body with known composition and described in an adequate physical theory is an example of [[Physics:Physical system|physical system]].

==In common usage==

An object is known by the application of [[Philosophy:Sense|sense]]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 scull encased in a rock may be considered an object because it is possible to determine the extent of the scull based on the properties of the material.

For a [[Rigid body|rigid body]], the boundary of an object may change over time by continuous [[Translation (geometry)|translation]] and [[Physics:Rotation|rotation]].  For a deformable body the boundary may also be continuously [[Physics:Deformation (mechanics)|deformed]] over time in other ways.

An object has an [[Philosophy:Identity|identity]].  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 [[Set (mathematics)|sets]] and [[Counting|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 objects identity is then based on the description of the system by continued identify 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 [[Physics:Classical mechanics|classical mechanics]] a physical body is collection of matter having properties including [[Physics:Mass|mass]], [[Physics:Velocity|velocity]], [[Physics:Momentum|momentum]] and [[Physics:Energy|energy]].  The matter exists in a volume of [[Three-dimensional space|three-dimensional]] space. This space is its [[Philosophy:Extension (metaphysics)|extension]].

Under Newtonian gravity 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 [[Physics:Continuum mechanics|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 which may be described internally by [[Physics:Pressure|pressure]] and [[Physics:Stress (mechanics)|mechanical stress]].

== Quantum mechanics ==

In [[Physics:Quantum mechanics|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 [[Wave function|probability distribution]] of finding the particle at a particular position.  There is a [[Physics:Uncertainty principle|limit to the accuracy with which the position and velocity may be measured]].  A particle or collection of particles is described by a [[Physics:Quantum state|quantum state]].

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

== String theory ==

In [[Physics:Particle physics|particle physics]], there is a debate as to whether some [[Physics:Elementary particle|elementary particle]]s are not bodies, but are [[Point (geometry)|points]] without [[Philosophy:Extension (metaphysics)|extension]] in physical space within [[Physics:Space-time|space-time]], or are always extended in at least one dimension of space as in [[Physics:String theory|string theory]] or M theory.

==In psychology==
In some branches of [[Social:Psychology|psychology]], depending on school of thought, a physical body is a [[Philosophy:Physical object|physical object]] with physical properties, as compared to mental objects.  In (reductionistic) [[Philosophy:Behaviorism|behaviorism]], a physical body and its 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 it is not a means only anymore, but its felt sense is a goal of its own. In [[Philosophy:Cognitive psychology|cognitive psychology]], physical bodies as they occur in [[Biology:Biology|biology]] are studied in order to understand the [[Philosophy:Mind|mind]], which may not be a physical body, as in [[Philosophy:Functionalism (philosophy of mind)|functionalist]] schools of thought.

==In philosophy==
{{Further|Philosophy:Ontology|Philosophy:Metaphysics}}

A physical body is an enduring object that exists throughout a particular [[Physics:Trajectory|trajectory]] of [[Space|space]] and orientation over a particular duration of [[Time|time]], and which is located in  the world of physical space (i.e., as studied by [[Physics:Physics|physics]]).  This contrasts with abstract objects such as [[Philosophy:Mathematical object|mathematical object]]s which do not exist at any particular time or place.
Examples are a [[Earth:Cloud|cloud]], a [[Medicine:Human body|human body]], a weight, a billiard ball, a table, or a [[Physics:Proton|proton]].  This is contrasted with abstract objects such as mental objects, which exist in the [[Philosophy:Mental world|mental world]], and [[Philosophy:Mathematical object|mathematical object]]s. Other examples that are '''''not''''' physical bodies are emotions, the concept of "[[Philosophy:Justice|justice]]", a feeling of hatred, or the [[Number|number]] "3".  In some philosophies, like the [[Philosophy:Idealism|Idealism]] of [[Biography:George Berkeley|George Berkeley]], a physical body '''''is''''' a mental object, but still has extension in the space of a visual field.

== See also ==
* [[Philosophy:Abstract object theory|Abstract object theory]]
* Deformable body
* [[Medicine:Human body|Human body]]
* [[Rigid body]]
* [[Philosophy:Non-physical entity|Non-physical entity]]

==References==
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