Physical body

In physics, a physical body or physical object (sometimes simply called a body or object) is an identifiable collection of matter, which may be more or less constrained to move together by translation or rotation, in 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 objects 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.