This topic should be dealt with only in so far as it can be made a subject for actual observation by the pupils. Children should learn to be thoughtful and observant and to do all kinds of work, manual as well as mental, intelligently.
MACHINES
(Consult The Ontario High School Physics, Chap. IX.)
Lever.—When a lever is used to lift a log, one end is placed under the log, a block called a fulcrum is placed under the lever as close as possible to the log, and then the workman pulls down on the outer end of the lever. For example, if the fulcrum is one foot from the log and ten feet from the man, the latter can raise ten pounds with a pull of one pound, but he has to move his end of the lever ten times as far as the log rises. Try it. See other examples in plough handles, see-saw, balance, scissors, wheel-barrow, pump-handle, handspike, crowbar, canthook, nut-crackers.
Rope and Pulley.—In the rope and pulley note that when the pulley is a fixed one, the only advantage is a changed direction of the rope. When the pulley is movable, the horse pulling will have only half the weight to draw if the pulley is single, one quarter if double, one sixth if triple, etc. Thus in the case of a common hay-fork the horse draws only half the weight of the hay, but he walks twice as far as the hay moves.
Cogs.—If one wheel has eighty cogs and the other ten, the latter will turn eight times to the former's once.
Belt.—When a belt runs over two wheels, one having, say, one fifth of the diameter of the other, the smaller will revolve five times for one revolution of the other.
Crank.—With a crank two feet long, one may turn a wheel twice as easily as with one one foot long, but the hand will move twice as far. If a wedge is two inches thick at the large end and ten inches long, a man may lift 1000 pounds by striking the wedge a 200-lb. blow.
Inclined plane.—If a plank twelve inches long has one end on the ground and the other on a cart four inches high, one man can roll up the plank the same weight that would require three men to lift, but he has to move the object three times as far.