Energy may be either potential or kinetic; potential energy means energy that is stored up and with nothing to act on, and for this reason it is called energy of position. The electric charge of a Leyden jar is potential energy but the moment it is released it makes a spark and becomes kinetic energy or energy of motion. Potential energy can be changed into kinetic energy and kinetic energy back again into potential energy with amazing freedom. Energy has a definite relation to velocity which means that when the speed of a moving body is increased its power to do work is also increased.
Like matter, energy cannot be destroyed, and so all of it taken together is called a constant quantity. When the energy stored up in a spring, or a battery, has been used the energy is not destroyed, though it may be very hard to find out where it has gone, but you may know that it has vanished in heat and in other forms of energy.
Work Against Friction.—The chief resistance which machines have to overcome is caused by friction. Since there is no such thing as a perfectly smooth surface friction is always present in machines and much energy must be spent in overcoming it. The energy wasted by friction is not destroyed but is transformed into another kind of energy and that is heat. When a marble is rolled over the surface of a table there is less friction between the two than when the marble slides across the table. Hence with ball bearings there is less friction than with cone bearings. (See [Appendix I].)
Forms of Energy.—There are nine forms of energy that you can make use of in your experiments and in your inventions, and these are:
Machines and the Principles of Machinery.—A machine is a contrivance of mechanical parts by which energy is transferred from one part to another. Beside the amount of energy required for doing useful work there must be an extra amount for overcoming the friction. Remember that no machine can either create energy or increase it, and, as you have seen, every machine wastes some energy in friction; this being true it must be clear then that it is impossible to make a machine which when once set in motion would continue to run forever, or at least until its parts were worn out. So don’t waste your energy in trying to invent a perpetual motion machine.
The Uses of Machines.—These are many and varied from a commercial point of view in that they are designed to do better, faster or cheaper work and sometimes all of these good qualities are found in a single machine.
From a mechanical point of view, though, a machine is used to
(1) Change one form of energy into another form, as steam into electricity.
(2) To make a slow moving, but powerful force produce a high speed or velocity, as in a sewing machine.