CHAPTER XVI 
FRICTION AND LUBRICATION 
2298, Sliding Friction—Coefficient of Friction.—Friction is the _ 
resistance which comes into action when one body is made to slide over _ 
another. The force of friction (F) is the least force, acting parallel to — 
the sliding surfaces of the bodies in contact, which will cause the one 
body to slide over the other. If Q is the mutual normal pressure — 
between the bodies in contact, the ratio F/Q is called the coefficient of — 
friction, and is denoted by ». The following table gives some values of — 
p. for moderate pressures and low speeds :— = 
Wood on wood, dry . 0:25 to 05 | Leather on wood, dry 0:3 to05 | 
mF »  soaped. O-1 ,, 0-2 | Leather on metal, dry . 03 . Ota 
me greased 0:02,, O-1 a 3 EROS 0°36 
Metal on wood, dry . 02° 5; 06 ¥5 2 greased 0°23 
Metal on metal, dry . 0715 ,, 03 a 53. ole 0°15 
Ss oiled inter- a 
mittently . ‘ 0:07 ,, 0°08 | Hemp ropes on metal, dry 0°2 to 0°34 | 
Metal on metal, oiled con- 
tinuously. . . . 0:04,, 0:06 me SR greased 0°15 
The foregoing values of » must be taken as approximate only. The — 
results of experiments on friction are very discordant. It has been found — 
that the coefficient of friction depends on the maierial of the sliding . 
bodies, the state of their surfaces as regards smoothness, the intensity of 
the pressure between the surfaces, the velocity of sliding, the nature and 
quantity of the lubricant and the manner in which it is applied, and — 
also on the temperature. . 
The friction at starting from rest or statical friction is greater than 
the friction of motion, and depends on the hardness of the bodies and — 
the length of time during which they have been in contact. 4 
The so-called laws of friction are—(1) The force of friction is directly ~ 
proportional to the pressure between the surfaces in contact. (2) The — 
force of friction is independent of the extent of the surfaces in contact. 
(3) The force of friction is independent of the velocity of sliding. These 
“laws” are approximately true when the intensity of the pressure between 
the surfaces is moderate, and when the speed of sliding is low. 
229. Relations between the Forces on a Sliding Body. _Conaidalal i 
first the case of a body A of weight W resting on a fixed horizontal plane. 
(Fig. 391). A is at rest under the action of two forces: (1) W, the — 
pressure of A on the plane; (2) R, the pressure of the planeon A. In 
this case R is obviously equal and ag to W. Suppose next that a — 
