Another simple machine, called an inclined plane, is a slope used to enable a small force, such as the strength of a man, to overcome the weight of a large body. When, for example, it is necessary to move heavy boxes, barrels, etc., from a sidewalk to a wagon or from a wagon to the sidewalk the teamster usually places a plank between the two distances, thus making an inclined plane and pushes the barrel or box onto the wagon. If a wagon bed is 4 ft. above the ground and a board 8 ft. long is placed against it, a man can then roll the barrel up the inclined plane with one-half the force he would have to exert when lifting, but in twice the time, as the distance covered is twice that of the vertical or upright height.

The mechanical power gained on an inclined plane is equal to the quotient obtained by dividing the length of the plane by the height. To illustrate: If a barrel weighing 300 lbs. is to be rolled onto a wagon 4 ft. from the ground and a plank 12 ft. long is used, a strength or force of 100 lbs. would balance the barrel, because the inclined plane is three times the perpendicular height. A slight force over the 100 lbs. would move the barrel.

Fig. 25.   Derrick.

Fig. 25. - Derrick.

Roads constructed to the tops of hills are either wound round and round, or made so broad that a person or driver of a vehicle can wind from side to side in climbing the hill. In building houses, an inclined plane in the form of a plank walk is used to facilitate the transit of wheelbarrows in and out of the building. The stairs of a house form a steep inclined plane on which the steps enable one to secure a firm footing.