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When a system is subjected to external forces, it generally undergoes a change in size or shape or both. We have thus far touched very lightly on such changes; for example, we have considered the change in length of an elastic spring and the change in volume of a gas when such systems were subjected to varying pressures. The changes produced in a system by the action of external forces depend upon the physical properties of the material of which the system is composed. A study of the properties of matter leads to information which is of practical value to both the physicist and the engineer, and also gives us some information about the internal forces which act between the constituent parts of the substance. In the final analysis these physical properties must be explicable in terms of the forces between the molecules of the substance and, in some cases, between the atoms of the substance.