Cindy Cook, age 11, of Pleasureville, Ky., for her question:
WHAT IS PERPETUAL MOTION?
It has been suggested that nuclear energy someday will provide us with. a source of perpetual motion. But the scientists say this probably isn't true. While uranium and other atomic fuels do contain a great deal of energy for their size, after the energy is used up it will be necessary that fresh fuel be added.
Perpetual motion is a movement of a hypothetical machine which being once set into motion should continue to go on forever by creating its own energy.
Despite the very earnest efforts of lots of inventors, almost all scientists say that there cannot now be perpetual motion or a device built that will produce work with no energy input.
Experiments made in efforts to achieve perpetual motion have brought scientists two laws of thermodynamics which summarize how all machines work. The first law says that it is impossible to create or destroy energy. The second says that heat can move only from a hot object to a colder object.
It has been suggested that a perpetual motion machine would have to be one of two types: a machine that would run forever without receiving energy from the outside or one that would run forever and produce work by taking energy from the sea or from the atmosphere.
if it were possible to avoid resistance, it might be possible to build a perpetual motion machine of the first type. But moving parts of all machines are subject to friction or some other type of resistance which slows down the machine.
In order to keep running, the machine would have to use energy to overcome this resistance. Without energy input, all machines finally stop. Therefore, scientists have decided that no machine creates energy.The second type of perpetual motion machine could be made to run forever if it were possible to use all of the energy in a large source, such as the sea or the atmosphere. But this type of machine would run only if all the energy of the randomly moving molecules in the source could be completely converted into useful work, and no machine has been able to do this.
Planets and their moons seem to have achieved perpet,ual motion but they move in an almost perfect vacuum.
Artificial satellites in orbit relatively close to the earth all have limited lifetimes because of atmospheric friction. The farther a satellite orbits from the earth, the longer its life expectancy. But scientists say that even satellites in orbit around the sun may eventually hit the sun, even though it might take millions of years.