FRICTION. When one body rubs against another as it moves, a certain force is felt to resist the motion. This resistance is called friction. As a considerable proportion of the motive-power in all operations is spent iu overcoming the F. of the parts of the machine upon one another, and is thus lost for the useful work, it is of great impor tance to understand the nature of this obstructive force, with a view to reduce it to the least possible amount. Accordingly, a great many careful experiments have been made on this subject, and the result is a number of precise aijd valuable facts or laws regard ing friction, which arc now considered certain and reliable. The more important may be thus stated and illustrated.
When a block of oak—say a cubic foot, which weighs about CO lbs.—is placed on a horizontal table of cast iron, the two surfaces being flat and smooth, it requires a force of nearly' the weight of the block, or 24 lbs., pulling horizontally, to make it slide along the table. This measures the F. between the two surfaces. Another block of the same size and shape laid on the same table, would require the same force to draw it; and if the two were laid side by side, and fastened together so as to become one block, it would evidently require double the force; or 48 lbs., to draw the double block; the amount of the F. being thus still of the weight, or of the pressure between the two surfaces. But suppose that, instead of being laid side by side, the second block were laid on the top of the first, what is to be expected? Here the weight is doubled as before, but the extent of rubbing surface remains unaltered; it would be natural, therefore, to expect that this would make a difference, and that, though the F.' would, of course, be increased, the increase would be less than in the former case. Experiment, however, shows that there is no difference, and that the F. is just double in both cases. In short, the unexpected and important fact is established, that, within certain limits, the F. of any two sus faces increases in proportion to the force with which they are pressed together, and is wholly independent of the extent of the surfaces in contact.
The amount of F. between two bodies is thus a constant fraction or proportion of the force with which they arc pressed against each other. This fraction differs for the different kinds of surfaces. Thus, between oak and cast iron, it is, as already stated, about -I, or more exactly, .38; for wrought iron on wrought iron (we speak at present of dry surfaces, without grease or unguent of any kind), it is .44; for brass upon cast iron, .22. This constant fraction (expressing the proportion between the pressure of two surfaces and their F.) is called the coefficient of for these two surfaces.
Another way of illustrating this law of F. is the following, which has an important hearing on the erection of structures, and on mechanics in general: Suppose a slab AB, in contact with another slab CD, of the same or of dif ferent material; and that a force PQ presses on AB obliquely. Let QR be the perpendicular to the two surfaces, and draw PR, PS parallel to AB and QR, thus.
resolving the force PQ into two forces, one, PS, press ing AB against CD, the other, PR or SQ, tending to make AB slide towards C. It will clearly depend upon the strength of F. between AB and CD, how far the force PQ may be made to decline from the perpendicu- a A. g S. ; p lar without actually causing the ouc body to slide on the other. Suppose that when the pushing force is ‘,\\ brought into the position P'Q, AB is just ready to slip on CD, and that it is a case of oak upon iron; then, since P'S' or R'Q is the force press ing the surfaces together, and P'R' or S'Q thh force tending to produce motion, P'R' will be of IN. The angle P'QR' is called the limiting angle of resistance of the two surfaces AB, CD; fur so long as the direction of the PQ'is within that angle, the F. of the surfaces will sustain it; but if the obliquity is greater, the surfaces will slip. This is true, independently of the extent of the surfaces in contact; and also of the amount of the pressure; for the stability depends upon the proportion of PR to RQ, and that is the same, whatever is the length of PQ, so long as its inclination is the same.