|Let us explore some of the ways material vibrations
have an immediate impact on the world around us. Most tangible substances
appear continuous to the eye. On close examination (with X rays) we now know
that solid matter is composed of myriads of molecules arranged in different
ways under the influence of intermolecular forces. The molecules themselves are
built from a relatively small number of distinct atomic constituents. Many
substances at room temperatures exhibit molecular patterns with long range
regularities composed of repeating structural units. The nature of the
constituents of a substance and the way in which the intermolecular forces are
distributed govern its physical properties. They are ultimately responsible for
the distinction between a lump of steel and a cotton thread. The ability of the
intermolecular forces to resist bulk deformation of a solid is called
elasticity and the forces that the body exerts across any plane in
resisting such deformations are known as elastic stresses.
Most fluids (liquids and gases) have different characteristics from solids since the molecular constituents are freer to move in space being constrained only by any boundaries of containing vessels. Elastic stresses cannot be resisted by static deformations of a fluid and inevitable cause it to move. The laws of Newton are unable to predict the detailed properties of molecules in solids, liquids or gases. However since a single gramme of most substances at room temperature and pressure contains so many trillions of molecules it is found that they can be described in terms of relatively few average macroscopic parameters such as density, pressure, temperature, rigidity, elasticity and viscosity. Such quantities are adequate for a description of materials according to the laws of Newton.