5. The radius of the wheel of a letter press is 6 in., the pitch of its screw is 1/4 in. What pressure is produced by a force of 40 lbs.? 6. The pitch of a screw of a vice is 1/4 in., the handle is 1 ft. long. what pressure can be expected if the force used is 100 lbs.? 7. A jackscrew is used to raise a weight of 2 tons. The bar of the jackscrew extends 2 ft. from the center of the screw. There are two threads to the inch. Find the force required. (6) FRICTION, ITS USES AND LAWS =130. Friction.=--Although often inconvenient and expensive, requiring persistent and elaborate efforts to reduce it to a minimum, friction has its uses, and advantages. Were it not for friction between our shoes and the floor or sidewalk, we could not keep our footing. _Friction is the resistance that must be overcome when one body moves over another._ It is of two kinds, _sliding_ and _rolling_. If one draws a block and then a car of equal weight along a board, the force employed in each case being measured by a spring balance, a large difference in the force required will be noticed, showing how much less rolling friction is than sliding friction. =131. Ways of Reducing Friction.=--(a) Friction is often caused by the minute projections of one surface sinking into the depressions of the other surface as one moves over the other. It follows, therefore, that if these projections could be made as small as possible that friction would be lessened. Consequently _polishing_ is one of the best means for reducing friction. In machines all moving surfaces are made as smooth as possible. In different kinds of materials these little ridges and depressions are differently arranged. (b) In Fig. 109 the friction between _R_ and _S_ would be greater than between _R_ and _T_. In _R_ and _S_ the surfaces will fit closer together than in _R_ and _T_. The _use of different materials will reduce friction_. The iron axles of car wheels revolve in bearings of brass. Jewels are used in watches for the same reason. (c) Another very common method of reducing friction is by the use of _lubricants_. The oil or grease used fills up the irregularities of the bearing surfaces and separates them. _Rolling friction_ is frequently substituted for sliding friction by the use of ball and roller bearings. These are used in many machines as in bicycles, automobiles, sewing machines, etc. (See Fig. 110.) [Illustration: FIG. 109.--The friction between _R_ and _S_ is greater than between _R_ and _T_.]