英文科學(xué)讀本 第六冊(cè)·Lesson 44 The Wedge

Lesson 44 The Wedge

The thing that must strike us most as we are becoming acquainted for the first time with the mechanical powers, said Mr. Wilson, "is the easy, natural way in which one machine has in every case suggested the next.

The last machine we studied was the inclined plane. There is just one point to remember now about that contrivance. It is employed to raise heavy bodies by actually pushing or dragging them up its slope, the plane itself remaining at rest. I have here a piece of hard wood, cut in the form of an inclined plane. I want you to take it in your hand, Fred, insert its thin edge under this large box on the table, and push it forward. What have you done? You have raised the box on the inclined plane, but the work has been done this time, not by pushing or pulling the box up the slope, but by forcing the inclined plane under the box. It is the inclined plane itself that moves, and not the body which we wish to raise.

We have here clearly a new machine. Let us examine it. Here are two such inclined planes exactly the same size. I will place them together base to base. The two, joined in this way, form a wedge. I have also an actual wedge all in one piece. We have only to draw a chalk line on one of its triangular sides, from the apex, perpendicular to the base, to show that it might be easily separated by the saw into two inclined planes exactly similar.

The wedge is, in fact, simply a pair of inclined planes, or, as we might say, a double inclined plane. Our next business is to learn how to make use of the wedge as a machine for accomplishing certain work. Let us turn our attention to this large cupboard, and suppose that it is necessary to raise the cupboard a few inches from the floor. It is too heavy to lift; how shall I do it?

Some of you will probably suggest the pulley, others the wheel and axle. Now, either of these might do very well if we wanted to lift it some distance, but no machine will accomplish what we want now so well as this simple little wedge. You shall take the wedge in your hand, Fred, insert its thin edge under the cupboard, and push it, as you did the little inclined plane under the box on the table. What is the matter?"

It will not move, sir, said Fred. "I cannot force it under the cupboard."

No, Fred, you are quite right. It will not move with all your pushing. Now take this hammer, and strike the wedge a few smart blows with it. Does the wedge move now?

Yes, sir; the blows of the hammer have driven the wedge between the bottom of the cupboard and the floor, and as the wedge moved under, it lifted the cupboard. The heavy body has been raised by the moving of the wedge.

Quite true, Fred, and if we compare the small exertion of striking the blows with the great weight of the body you have raised, we shall see that this little machine gives great mechanical advantage. There is a point in connection with the wedge worthy of notice. As the power is always applied to a wedge by a blow with a hammer, and not by pulling or pushing, it is not easy to calculate exactly the mechanical advantage with this machine as we have done with others.

I want you to notice, too, that the cupboard still remains raised up, just where the wedge carried it. Why does not the weight of the body, pressing downwards, force the wedge out? Perhaps I had better tell you. It would do so if the wedge and the bottom of the cupboard itself were perfectly smooth; in other words, if there were no friction. It is the friction between the two that grips, or holds the wedge in its place, every time the blow of the hammer has sent it a little farther in. If it were not for this friction, the wedge would rebound and fly out after the blow had been struck.

One of the commonest uses of the wedge is for splitting great blocks of wood. Axe-heads, knives, chisels, nails, planes—indeed, most cutting implements—are applications of the same principle, for they are all wedges, thin at one edge and thicker at the other.