When meteorite flashes into the earth's atmosphere, why does it flare up? What holds a nail in a board? How is a railroad embankment held up? Why does a match work?
Any scientist might answer these questions with a single word: friction. Despite its familiarity, men have spent centuries trying to unlock the secrets of friction. Researchers are still probing the effects friction produces to answer their own questions: how to get along in spite of friction; how to put it to work; how to overcome it.
It is not at all surprising that the first known definition of friction was set down by master-of-all-trades Leonardo da Vinci. Da Vinci observed that "the friction made by the same weight will be of equal resistance although the contact may be of different breadths and lengths" and that "friction produces double the amount of effort if the weight be doubled."
But long before the Italian genius, men were grappling with the effects of friction. As early as the eleventh century men reasoned that if a machine could be invented which would operate entirely on its own power, they would have, in effect, a "perpetual motion" machine, one that would defeat the effects of friction. Such a machine, in which the output would equal the input, would be a triumph equivalent to finding the philosopher's stone that would transmute base metal into gold.
One of the earliest recorded attempts to defeat friction was a perpetual motion wheel devised by Wilars de Honnecourt, a thirteenth century architect. His invention employed weights which descended on one side of a wheel and at the same time hoisted weights on the opposite side, up and over the midpoint of the wheel, where they in turn became the descending weights. Theoretically, the wheel, once started, was supposed to turn forever without help from an outside power. Friction, however, predetermined the wheel's failure.
The failure of de Honnecourt's wheel—and thousands more just like it—did not discourage inventors. The impossibility of perpetual motion on mathematical and mechanical grounds was completely ignored, and hardly a year passed without a scientist somewhere putting together a strange looking contraption that, in the end, as one exasperated inventor put it, "will not go even though I have labored over it the past twelve months."
It was not until the seventeenth century when the physical sciences had evolved to a point where the idea of perpetual motion ceased to be a logical possibility that the claims of inventors for it began to disappear. The development of the laws of friction, along with the doctrine of the conservation of energy, lead to more practical considerations in physics and mechanics.
Da Vinci's ideas about friction remained buried in his notebooks for some 200 years, until a French engineer, Amontons, reformulated them. Then, in 1781, C. A. Coulomb verified Amontons' observations in the laboratory and stated them in the form of the laws of friction as they are known today. The laws are quite simple. The two principle laws have been only slightly modified since Coulomb first proposed them.
The first of these laws states that the friction is independent of the area of the solids. If, for example, a hook is pulled along a table the factional force is the same whether the book is lying flat or standing on its end. The second law states that the friction is proportional to the load between the two surfaces. If, to use the same example, the load is doubled by placing a second book on top of the first, the force required to cause sliding is twice as great.
Examples of sliding friction at work are represented by countless phenomena in everyday life. Walking, the nails in the walls of houses, automobile brakes, earthworks for railroad embankments, etc. To state the matter in reverse, such things as matches, nails, and screws would be useless were it not for the existence of sliding friction.
Coulomb concluded that the nature of friction might depend on the interlocking of surface irregularities, and the "drag" we feel would be due to the work required to lift the load over the irregularities. Modern researchers are seeking an answer to Coulomb's question: "What actually happens on the surface during the sliding of one object over another?" Some of the answers are interesting.
They have found, for example, that no surface, no matter how highly polished, is without irregularities. The smoothest surface that precision engineering can obtain still contains microscopic rough spots with depths equal to l/500th the diameter of a human hair. The rough spots, when magnified by an electron microscope, resemble a countryside filled with ravines and cliffs.
When the surfaces are put together, the effect is somewhat like turning Switzerland upside down and putting it on top of Austria. The metals or solids rest only at their points of contact so that the actual area of real contact is quite small. The pressure or load between the two surfaces gradually crushes the points of contact and causes them to flow together, plastically.
Friction is one of the problems that space scientists face in designing motors or mechanical systems for outer space. The parts of a motor compressor that function efficiently at ground level start to "cold weld" (adhere) through friction when they reach the high vacuums of outer space.
Temperatures are produced by friction not only by the movement of one body against another, but the movement of a solid against the air itself. Space capsules or missiles re-entering the earth's atmosphere generate enormous amounts of heat by literally "rubbing" against the atmosphere. The temperature at re-entry is 2,600 degrees F, enough to incinerate a capsule unless some means of cooling the surface is available. The most successful method to date has been to sacrifice a portion of the surface material of the nose cone by allowing it to vaporize in order to cool the heat shield.
At ground level, the temperature produced by the friction of one solid rubbing over another varies considerably, depending on the speed of contact, pressure exerted, and heat conductivity of the metal itself. The "points" that come into contact may become white hot, generating split-second heat up to 2,732 degrees F., even though the metal appears to remain cool.
In some cases where the rubbing surfaces have been brought together at speeds of more than 1,000 miles per hour, researchers have found that a thin film of metal was melted between the two surfaces, the film itself being a form of lubricant. Scientists suggest that this film may explain what happens in the act of polishing. Rubbing smears a melted layer that bridges and fills up scratches. When the surface cools, it forms a slick micro-crystalline layer.
The lubrication of sliding solid objects presents special problems when the temperature at the point of contact is exceedingly high. Quite often a temperature flash at a rubbing spot leads to the failure of a lubricant just when its protective properties are most needed. To counteract this requires the right combination of lubricants for a particular pair of surfaces, speed, load, and melting point. Finding the right combination constitutes the exacting art of the lubrication engineer.
The problems of lubrication where such machinery as piston engines and motor compressors are concerned is a problem of surfaces separated by relatively thick films of lubricant. The oil is actually a "buffer" that prevents the two surfaces from coming into direct contact. In automobile engines, for example, the viscosity of the oil (its thickness or thinness) is adjusted to suit the requirements of the moving parts. If the oil is too thin, the friction between moving parts will damage the machine. If the oil is too thick, the lubricant will clog the action.
A revolutionary step in reducing friction and wear was taken in the eighteenth century with the invention of ball bearings. It has always been obvious since the birth of the wheel that it is much easier to roll an object than to slide it. Formerly such devices as shafts, or wheel axles, slid or rotated in their sockets around each other with correspondingly high frictional losses. Ball bearings allowed them to roll past each other with a great reduction in friction loss.
The use of ball bearings, better lubricants and the knowledge gained from a study of sliding surfaces have made it possible to run machines faster and hotter. However, to the engineer concerned with practical applications, it is neither the source of friction or the loss in power with which he is primarily concerned, although both are major problems. It is the damage in the form of wear or seizure of the vital parts of the machine that most concerns him. This factor more than anything else limits the design and shortens the working life of machines. It is the answer to this problem that physics in its study of friction is attempting to solve.
The complete answer to everything that happens when a meteor flashes through the earth's atmosphere is still un known. But today man-made meteors, sparkling with the same brilliance as nature's meteors, are flashing through the heavens. They may find the answer.
