Physics

Ratio of the mass of a substance to its volume, expressed, for example, in units of grams per cubic centimeter or pounds per cubic foot. The density of a pure substance varies little from sample to sample and is often considered a characteristic property of the substance.

State of balance. When a body or a system is in equilibrium, there is no net tendency to change. In mechanics, equilibrium has to do with the forces acting on a body.

The study of the mechanics of fluids. Fluid statics is concerned with the properties of fluids at rest. Fluid dynamics considers the properties peculiar to moving fluids.

Resistance offered to the movement of one body past another body with which it is in contact. In certain situations friction is desired. Without friction the wheels of a locomotive could not "grip" the rails nor could power be transmitted by belts.

Arrangement of moving and stationary mechanical parts used to perform some useful work or to provide transportation. From a historical perspective, many of the first machines were the result of human efforts to improve war-making capabilities; the term engineer at one time had an exclusively military connotation.

Branch of physics concerned with motion and the forces that tend to cause it; it includes study of the mechanical properties of matter, such as density, elasticity, and viscosity.

In mechanics, the quantity of motion of a body, specifically the product of the mass of the body and its velocity. Momentum is a vector quantity; i.e., it has both a magnitude and a direction, the direction being the same as that of the velocity vector.

In his Principia Newton (1687) stated the three fundamental laws of motion, which are the basis of Newtonian mechanics.

In mechanics, ratio of the force acting on a surface to the area of the surface; it is thus distinct from the total force acting on a surface. A force can be applied to and sustained by a single point on a solid. However, a force can only be sustained by the surface of an enclosed fluid, i.e., a liquid or a gas.

Quantitative study of systems consisting of a large number of interacting elements, such as the atoms or molecules of a solid, liquid, or gas, or the individual quanta of light (see photon) making up electromagnetic radiation.

From The Penguin Dictionary of Physics
A quantity with magnitude and direction. It can be represented by a line whose length is proportional to the magnitude and whose direction is that of the vector, or by three components in a rectangular coordinate system.

Mean (weighted average) of the masses of all the naturally occurring isotopes of a chemical element, as contrasted with atomic mass, which is the mass of any individual isotope.

From The Columbia Encyclopedia
Avogadro's number (ävōgä´drō) [for Amedeo Avogadro], number of particles contained in one mole of any substance.

In physics, an ideal black substance that absorbs all and reflects none of the radiant energy falling on it . . . Since a black body is a perfect absorber of radiant energy, by the laws of thermodynamics it must also be a perfect emitter of radiation.

From The Penguin Dictionary of Science
A value that appears as a constant of proportionality in a fundamental physical law, independent of location in the Universe. There are three broad categories of fundamental constant

Theory describing the physical properties of matter in terms of the behaviour - principally movement - of its component atoms or molecules. It states that all matter is made up of very small particles that are in constant motion, and can be used to explain the properties of solids, liquids, and gases, as well as changes of state.

Weight of a molecule of a substance expressed in atomic mass units (amu). The molecular weight may be calculated from the molecular formula of the substance; it is the sum of the atomic weights of the atoms making up the molecule.

A system of mechanics applicable at distances of atomic dimensions, 10−10 m or less, and providing for the description of atoms, molecules, and all phenomena that depend on properties of matter at the atomic level.

Modern physical theory concerned with the emission and absorption of energy by matter and with the motion of material particles; the quantum theory and the theory of relativity together form the theoretical basis of modern physics.

From Dictionary of Energy
Nuclear: a fundamental physical constant used in studies of the spectrum of a substance.

Physical principle, enunciated by Werner Heisenberg in 1927, that places an absolute, theoretical limit on the combined accuracy of certain pairs of simultaneous, related measurements. The accuracy of a measurement is given by the uncertainty in the result; if the measurement is exact, the uncertainty is zero.