7. MATTER IS FRICTIONABLE.
A bullet shot into the air has its velocity continuously reduced by the air, to which its energy is imparted by making it move out of its way. A railway train is brought to rest by the friction brake upon the wheels. The translatory energy of the train is transformed into the molecular energy called heat. The steamship requires to propel it fast, a large amount of coal for its engines, because the water in which it moves offers great friction—resistance which must be overcome. Whenever one surface of matter is moved in contact with another surface there is a resistance called friction,
the moving body loses its rate of motion, and will presently be brought to rest unless energy be continuously supplied. This is true for masses of matter of all sizes and with all kinds of motion. Friction is the condition for the transformation of all kinds of mechanical motions into heat. The test of the amount of friction is the rate of loss of motion. A top will spin some time in the air because its point is small. It will spin longer on a plate than on the carpet, and longer in a vacuum than in the air, for it does not have the air friction to resist it, and there is no kind or form of matter not subject to frictional resistance.