This article does not cite any sources . (July 2012) (Learn how and when to remove this template message)
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.
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.
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.
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.
In particle physics, there is a debate as to whether some elementary particles are not bodies, but are points without extension in physical space within space-time, or are always extended in at least one dimension of space as in string theory or M theory.
In some branches of psychology, depending on school of thought, a physical object has physical properties, as compared to mental objects. In (reductionistic) 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 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 are studied in order to understand the mind, which may not be a physical body, as in functionalist schools of thought.
A physical body is an enduring object that exists throughout a particular trajectory of space and orientation over a particular duration of time, and which is located in the world of physical space (i.e., as studied by physics). This contrasts with abstract objects such as mathematical objects which do not exist at any particular time or place. Examples are a cloud, a human body, a weight, a billiard ball, a table, or a proton. This is contrasted with abstract objects such as mental objects, which exist in the mental world, and mathematical objects. Other examples that are not physical bodies are emotions, the concept of "justice", a feeling of hatred, or the number "3". In some philosophies, like the Idealism of George Berkeley, a physical body is a mental object, but still has extension in the space of a visual field.
The holographic principle is a tenet of string theories and a supposed property of quantum gravity that states that the description of a volume of space can be thought of as encoded on a lower-dimensional boundary to the region—such as a light-like boundary like a gravitational horizon. First proposed by Gerard 't Hooft, it was given a precise string-theory interpretation by Leonard Susskind who combined his ideas with previous ones of 't Hooft and Charles Thorn. As pointed out by Raphael Bousso, Thorn observed in 1978 that string theory admits a lower-dimensional description in which gravity emerges from it in what would now be called a holographic way. The prime example of holography is the AdS/CFT correspondence.
Mass is both a property of a physical body and a measure of its resistance to acceleration when a net force is applied. An object's mass also determines the strength of its gravitational attraction to other bodies.
Mechanics is the area of physics concerned with the motions of macroscopic objects. Forces applied to objects result in displacements, or changes of an object's position relative to its environment. This branch of physics has its origins in Ancient Greece with the writings of Aristotle and Archimedes. During the early modern period, scientists such as Galileo, Kepler, and Newton laid the foundation for what is now known as classical mechanics. It is a branch of classical physics that deals with particles that are either at rest or are moving with velocities significantly less than the speed of light. It can also be defined as a branch of science which deals with the motion of and forces on bodies not in the quantum realm. The field is yet less widely understood in terms of quantum theory.
M-theory is a theory in physics that unifies all consistent versions of superstring theory. The theory was first conjectured by Edward Witten at a string theory conference at the University of Southern California in the spring of 1995. Witten's announcement initiated a flurry of research activity known as the second superstring revolution.
Physics is the natural science that studies matter, its motion and behavior through space and time, and the related entities of energy and force. Physics is one of the most fundamental scientific disciplines, and its main goal is to understand how the universe behaves.
Physical science is a branch of natural science that studies non-living systems, in contrast to life science. It in turn has many branches, each referred to as a "physical science", together called the "physical sciences".
The following outline is provided as an overview of and topical guide to physics:
Space is the boundless three-dimensional extent in which objects and events have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum known as spacetime. The concept of space is considered to be of fundamental importance to an understanding of the physical universe. However, disagreement continues between philosophers over whether it is itself an entity, a relationship between entities, or part of a conceptual framework.
In physics, action at a distance is the concept that an object can be moved, changed, or otherwise affected without being physically touched by another object. That is, it is the nonlocal interaction of objects that are separated in space.
In theoretical physics, the anti-de Sitter/conformal field theory correspondence, sometimes called Maldacena duality or gauge/gravity duality, is a conjectured relationship between two kinds of physical theories. On one side are anti-de Sitter spaces (AdS) which are used in theories of quantum gravity, formulated in terms of string theory or M-theory. On the other side of the correspondence are conformal field theories (CFT) which are quantum field theories, including theories similar to the Yang–Mills theories that describe elementary particles.
Absolute space and time is a concept in physics and philosophy about the properties of the universe. In physics, absolute space and time may be a preferred frame.
A point particle is an idealization of particles heavily used in physics. Its defining feature is that it lacks spatial extension: being zero-dimensional, it does not take up space. A point particle is an appropriate representation of any object whenever its size, shape, and structure are irrelevant in a given context. For example, from far enough away, any finite-size object will look and behave as a point-like object. A point particle can also be referred in the case of a moving body in terms of physics.
Mathematics encompasses a growing variety and depth of subjects over history, and comprehension requires a system to categorize and organize the many subjects into more general areas of mathematics. A number of different classification schemes have arisen, and though they share some similarities, there are differences due in part to the different purposes they serve. In addition, as mathematics continues to be developed, these classification schemes must change as well to account for newly created areas or newly discovered links between different areas. Classification is made more difficult by some subjects, often the most active, which straddle the boundary between different areas.
In physics, the degree of freedom (DOF) of a mechanical system is the number of independent parameters that define its configuration. It is the number of parameters that determine the state of a physical system and is important to the analysis of systems of bodies in mechanical engineering, aeronautical engineering, robotics, and structural engineering.
The general formal ontology (GFO) is an upper ontology integrating processes and objects. GFO has been developed by Heinrich Herre, Barbara Heller and collaborators in Leipzig. Although GFO provides one taxonomic tree, different axiom systems may be chosen for its modules. In this sense, GFO provides a framework for building custom, domain-specific ontologies. GFO exhibits a three-layered meta-ontological architecture consisting of an abstract top level, an abstract core level, and a basic level. Primarily, the ontology GFO:
Soft-body dynamics is a field of computer graphics that focuses on visually realistic physical simulations of the motion and properties of deformable objects. The applications are mostly in video games and films. Unlike in simulation of rigid bodies, the shape of soft bodies can change, meaning that the relative distance of two points on the object is not fixed. While the relative distances of points are not fixed, the body is expected to retain its shape to some degree. The scope of soft body dynamics is quite broad, including simulation of soft organic materials such as muscle, fat, hair and vegetation, as well as other deformable materials such as clothing and fabric. Generally, these methods only provide visually plausible emulations rather than accurate scientific/engineering simulations, though there is some crossover with scientific methods, particularly in the case of finite element simulations. Several physics engines currently provide software for soft-body simulation.
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 particles, and in everyday as well as scientific usage, "matter" generally includes atoms and anything made up of them, and any particles that act as if they have both rest mass and volume. However it does not include massless particles such as photons, or other energy phenomena or waves such as light or sound. Matter exists in various states. These include classical everyday phases such as solid, liquid, and gas – for example water exists as ice, liquid water, and gaseous steam – but other states are possible, including plasma, Bose–Einstein condensates, fermionic condensates, and quark–gluon plasma.
Physics deals with the combination of matter and energy. It also deals with a wide variety of systems, about which theories have been developed that are used by physicists. In general, theories are experimentally tested numerous times before they are accepted as correct as a description of Nature. For instance, the theory of classical mechanics accurately describes the motion of objects, provided they are much larger than atoms and moving at much less than the speed of light. These "central theories" are important tools for research in more specialized topics, and any physicist, regardless of his or her specialization, is expected to be literate in them.
This glossary of physics is a list of definitions of terms and concepts relevant to physics, its sub-disciplines, and related fields, including mechanics, materials science, nuclear physics, particle physics, and thermodynamics.
A surface, as the term is most generally used, is the outermost or uppermost layer of a physical object or space. It is the portion or region of the object that can first be perceived by an observer using the senses of sight and touch, and is the portion with which other materials first interact. The surface of an object is more than "a mere geometric solid", but is "filled with, spread over by, or suffused with perceivable qualities such as color and warmth".