Physical object

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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.^{[citation needed]} The boundary surface must be defined and identified by the properties of the material, although it 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 is 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

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 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.

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

An object has an 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 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.

A living thing may be an object, and is distinguished from non-living things by the designation of the latter as inanimate objects. Inanimate objects generally lack the capacity or desire to undertake actions, although humans in some cultures may tend to attribute such characteristics to non-living things.^[1]

Classical mechanics

In classical mechanics a physical body is collection of matter having properties including mass, velocity, momentum and energy. The matter exists in a volume of three-dimensional space. This space is its extension.

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 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 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

In psychology

In some branches of

A physical body is an enduring object that exists throughout a particular

• Abstract object theory
• Deformable body
• Human body
• Non-physical entity
• Physical model
• Rigid body
• Ship of Theseus, a thought experiment about an object's identity over time

References