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In common usage, 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 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 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.

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.

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, several interacting smaller bodies (particles or other), and continuous media.

The common conception of physical objects includes that they have extension in the physical world, although there do exist theories of quantum physics and cosmology which may challenge[how?] 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 a point. A physical body as a whole is assumed to have such quantitative properties as mass, momentum, electric charge, other conserving 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 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 rigid body, the boundary of an object may change over time by continuous translation and rotation. For a deformable body the boundary may also be continuously deformed over time in other ways.

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 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 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.

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 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 pressure and mechanical stress.