In physics, a physical body, or simply body, is something that has mass and takes up space . In classical physics , a body is usually restricted by an identifiable limit. Bodies are made of matter . A body can exist in one of the physical states, e.g. B. solid, liquid or gaseous, but it can also be composed of several components with possibly different physical states. In classical physics, the following applies: where there is one body, there cannot be another. In quantum physics , too , a body is something that has mass and takes up space. On the other hand, the concept of limitation is hardly applicable since limitation cannot be localized at will.
Conceptually, a distinction is made between the countable and divisible body and the non-countable and indivisible matter . For example, “snowflake”, “board” or “drop” are names for bodies, while “ice”, “wood” and “water” are names for the respective substances.
Liquid and gaseous bodies are collectively called fluids . They do not have a specific shape and usually adapt to the vessel walls. The main difference is that gases can be compressed while liquids have an almost constant volume . A gas usually completely fills the available space, while a liquid forms a surface. If a body is in a fluid, it experiences a buoyancy according to the Archimedean principle . The behavior of immobile fluids is described by fluid statics , the behavior of moving fluids by fluid dynamics .
If a gaseous body condenses to a liquid or solid, its density increases suddenly, typically by three orders of magnitude . On the other hand, it changes only slightly when it solidifies . That is why liquids and solids are combined as condensed matter and contrasted with gases.
Solid bodies have a fixed shape, but they can be deformed by the action of external forces . If the deformation goes back completely after the action of the force, then one speaks of an elastic deformation , otherwise of a plastic deformation , which is an irreversible process . In many applications, the deformations of a solid body, if they are only minor, are also neglected. The rigid body model is then used . In order to clearly describe its position in space, it is sufficient to give the coordinates of three of its points. If the properties of a solid, in particular its composition and density, are constant in the entire volume, one speaks of a homogeneous body.
If, in addition to the deformation, the rotation of the body can also be neglected (e.g. because the expansion of the body is very small or because the rotation is impossible due to constraints ), one can also correctly describe its position and its movement by reduced it to a single mass point . One imagines the entire mass united in the center of gravity of the body, which is determined by the weighted mean of the masses of all mass points or volume elements from which the body is built. Strictly speaking, the individual mass point is no longer a physical body because it has no spatial extension. As Newton's laws describe, an external force causes an acceleration in the direction of the force, which is greater, the greater the force and the smaller the mass of the mass point.
The movement of a mass point always means a change in place over time . Corresponding to the three spatial dimensions, a mass point has three degrees of freedom of translation . If a rigid body consists of more than one mass point, it can also rotate around any axis and thus receive up to three degrees of freedom of rotation . The dynamic behavior is no longer determined solely by the mass of the body, but also by the spatial distribution of the mass, which is indicated by the inertia tensor . Forces that act on a body cause (as with the mass point) translational acceleration. If they grip eccentrically, i. H. outside the center of gravity, they also cause rotational accelerations.
In the case of a rigid body, finally, the summation over a finite number of mass points is replaced by the integration over infinitesimally small volume elements . The rigid body has six degrees of freedom : three for translation and three for rotation.
When modern science emerged in the early 17th century, René Descartes represented a purely geometric term. Accordingly, the body is defined solely by its volume and shape and has no other properties. The body should only be able to trigger effects through direct contact with another body, i.e. through a shock. In contrast, Isaac Newton based the mechanics named after him on the definition that a body is a certain amount of matter. He used the term body synonymously with mass and understood by it the total number of material particles that are enclosed in a volume. With the help of the concept of force , which he also newly created , he analyzed the movements of bodies in the sky and on the earth and found that an attraction must act between any two bodies, even at a great distance, namely gravity . Although Newton himself did not want to make any statements ("hypotheses") about the origin of this force, this force effect was understood as starting from the bodies and as a property of the bodies. Gottfried Wilhelm Leibniz and others criticized this as a relapse into the pre-scientific era, when attempts were made to explain the processes observed in nature by attributing all kinds of hidden ( occult ) properties to the bodies - a view that Newton vehemently rejected.
In the further expansion of mechanics and their application to liquids and gases, the use of the word body was expanded accordingly. At the beginning of the 20th century, new radionuclides found in radiochemical experiments, initially only identified by their measurable half-life, were referred to as radioactive bodies .
- Richard S. Westfall: Force in Newton's Physics: The Science of Dynamics in the Seventeenth Century . American Elsevier, New York 1971. , chap. 2
- Isaac Newton: Philosophiae naturalis principia mathematica. Vol. 1: Tomus Primus. London 1687, Definitio I ( digitized version ), see also Newton: Opticks , Book III, Query 31 (complete document)
- Volkmar Schüller: The Leibniz-Clarke correspondence . Akademie-Verlag, Berlin 1991. , letter to A. Conti dated December 6, 1715