Perpetual motion is an idea which people from ancient times tried to build into some kind of machine. The idea was that a properly built machine would run forever without having any constant supply of energy from outside itself. Thus, it would run itself while producing its own power to continue running.
The scientific view today is that a real perpetual motion machine is a practical impossibility. The reasoning is as follows.
Any machine starts to operate when supplied with some definite amount of energy of the proper type (for which the machine was designed) and of the proper "energy order-level." High-level energy forms include those mechanical motions of an engine drive-rod or a steady current of electricity or of a hot object sending its heat (molecular motion) into a colder object. The colder object has the lowest order of energy of the whole series named.
As the machine starts up, the moving parts rub together and wear away. Thus they waste some of the original high-level motion and spread it about as worn machine fragments and low-level heat. In short, friction and heat loss are the two ever-present conquerors of perfect use of energy and thus of perpetual motion. The second law of thermodynamics, states that the heat in a material cannot be completely changed into mechanical energy—except if the machine could work at absolute zero (—460 °F., — 273.1 °C.). absolute zero temperature has never been reached.
The planets and natural satellites, such as the moon, do seem to travel about their central bodies perpetually, for they move in the near-vacuum of space and undergo little or no friction. The main friction-like forces on satellites are those made by space debris—meteors, or comets. Such debris— or large-sized collisions—might sometime end the perpetual motion of even these bodies.
One of the many proposed perpetual motion machines is that sketched above, right. It has three springs, one of which is supported by two upright rods. The other two have metal spheres at their lower ends.
The system is started by introducing energy into sphere A; that is, it is set to vibrating up and down. Eventually sphere B will vibrate and A will come to rest.
This process will repeat itself for quite some time. Then why will it fail in perpetual motion? There will always be some air friction; and even in a vacuum, there will also be the internal friction and heat loss of the molecules in the springs themselves. Without adding more outside energy then, this machine will finally run down.
The perpetual motion idea has been valuable since it has led men to build better machines—with better lubrication and finer parts, such as ball bearings.
