"I will adjust the plane so as to make its height exactly half the length of its slope, and that will give us an incline of 30°. To the top of the slope a small pulley shall be fixed, and I will attach one end of the cord which passes over the pulley to some smooth object standing on the plane. I have chosen for our object this little toy truck, which runs on smooth, polished wheels; and I have loaded the truck to make it weigh exactly 2 lbs. Now, Fred, you shall hold the cord, while I attach to the other end of it a 1 lb. weight.
When that is done we will leave the truck to take care of itself, and we see that the 1 lb.weight is sufficient to balance it on the slope. A half-ounce weight more hung on the cord will make the truck travel up the inclined plane.
"We may alter the inclination of the plane now, and we shall find that as we vary the slope so the power will vary. The little truck will not be supported by the same power on different slopes. Thus, if we make the height one-third the length of the slope, the truck will be supported by one- third its weight; if the height be one-fourth the length, it will require only one-fourth its weight to support it.
"In other words, the longer we make the slope ascompared with the height, the greater is the mechanical advantage. It is important to remember that we have made use of the pulley only to enable us to measure the force required. Whether the body be pushed upwards from behind, or pulled up by means of a cord, the force necessary to accomplish it is the same in each case.
"We will next glance at a few practical applications of the inclined plane.
"We have already had something to say about the drayman"s ladder. Next time you see one, notice how smooth its working surfaces are. You are now prepared, of course, to tell what kind of ladder is most advantageous to him-a long or a short one-and to give your reasons for what you say.
"We often see an inclined plane used for transferring a great block of stone into a cart. The rough surface of thestone, in addition to its weight, would, however, prevent it from moving even on the inclined plane. Hence, in order to overcome some of the friction, three or four rollers are placed under the block, and, as these roll forward, they carry the block with them, the hindmost one being transferred to the front from time to time, as it is disengaged.
"In the construction of bridges a great deal of thought and care must be given to the length of the approaches on either side. These approaches slope upwards towards the middle of the bridge, and the slope must be made sufficiently long in comparison with the height, so as not to cause too great a strain on the horses which will have to draw heavy loads over the bridge.
"The horse travels with its burden along a level road with comparative ease, because the weight rests upon the wheels of the cart, and it has only to overcome the friction between the wheels and the rough surface of the road. As soon, however, as it begins to mount a hill, the weight is thrown behind the wheels, and there is now, in addition to the friction, another force to contend against, namely, the force of gravity, due to the weight of the load, and it is plain that this must increase with the increase of the slope.
"The road up a steep hill is, for the same reason, usually made to wind round and round the hill, in order to give a longer slope, and so render the ascent easy."
