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SOME PROPERTIES OF SOLID BODIES—INERTIA—MOTION—FRICTION—THE PENDULUM—EQUILIBRIUM.

Those who have followed us through the preceding pages have now, we hope, some ideas upon Gravity and the Forces of Nature. In speaking of Forces we said “Force was a cause of Motion.” Let us now consider Inertia, and Motion with its accompanying opponent, Friction.

Fig. 29.—Shock communicated by elasticity.

Inertia is the passiveness of Matter. This perfect indifference to either rest or motion makes the great distinction between living and lifeless matter. Inertia, or Vis Inertia, is this passiveness. Now, to overcome this indifference we must use force, and when we have applied force to matter we set it in motion; that is, we move it. When we move it we find a certain resistance which is always proportionate to the force applied. In mechanics this is termed Action, and Reaction, which are always equal forces acting in opposite directions. This is Newton’s law, and may be explained by a “weight” on a table, which presses against the table with the same force with which the table presses against the “weight”; or when you strike a ball, it strikes the hand with the same force.

We can communicate motion by elasticity. For instance, if we place a number of coins upon a table touching each other and in a straight line, and strike the last coin of the line by pushing another sharply against it, the piece at the opposite extremity will slip out of its place from the effect of the shock transmitted by the coin at the other end (fig. 29).

Fig. 30.—Experiment to illustrate inertia.

When two forces act upon a body at the same time, it takes a direction intermediate. This is known as the resultant. The enormous forces exercised by the heavenly bodies will be treated of later. We will first consider Inertia.

There are several experiments relating to the subject of Inertia which may be performed. I once witnessed one quite accidentally when taking a walk.

Fig. 31.—Another experiment on the same subject.

I was one day passing the Observatory at Paris, when I noticed a number of people collected round a professor, who after executing several juggling tricks, proceeded to perform the curious experiment I am about to describe. He took a broomstick and placed it horizontally, passing the ends through two paper rings. He then asked two children to hold the paper rings by means of two razors, so that the rings rested on the blade. This done, the operator took a stout stick, and, with all his strength, struck the broomstick in the centre; it was broken into shivers, but the paper rings were not torn in the least, or even cut by the razors! One of my friends, M. M——, a painter, showed me how to perform this experiment as represented in the illustration (fig. 30). A needle is fixed at each end of the broomstick, and these needles are made to rest on two glasses, placed on chairs; the needles alone must be in contact with the glasses. If the broomstick is then struck violently with another stout stick, the former will be broken, but the glasses will remain intact. The experiment answers all the better the more energetic the action. It is explained by the resistance of inertia in the broomstick. The shock suddenly given, the impulse has not time to pass on from the particles directly affected to the adjacent particles; the former separate before the movement can be transmitted to the glasses serving as supports.8

Fig. 32.—Extracting a “man” from a pile of draughts without overturning the pile.

The experiment represented in fig. 31 is of the same nature. A wooden ball is suspended from the ceiling by a rather slender thread, and a similar thread is attached to the lower end of the ball. If the lower thread is pulled forcibly it will break, as shown in the illustration; the movement communicated to it has not time to pass into the ball; if, on the contrary, it is pulled very gradually and without any shock, the upper thread instead will break, because in this case it supports the weight of the ball. Motion is not imparted simultaneously to all parts of a body, but only to the particles first exposed to a blow, for instance. One might multiply examples of this. If a bullet be shot from a gun, it will make a round hole in a piece of wood or glass, whilst if thrown by the hand—that is to say, with much less force,—it will shiver the wood or the pane of glass to pieces. When the celerity of the motive force is very great, the particles directly affected are disturbed so quickly that they separate from the adjacent particles before there is time for the movement to be communicated to the latter.

It is possible, for the same reason, to extract from a pile of money a piece placed in the middle of the pile without overturning the others. It suffices to move them forcibly and quickly with a flat wooden ruler. The experiment succeeds very well also if performed with draughtsmen piled up on the draught-board (fig. 32).

Fig. 33.—Calling out a sixpence from the glass.

Fig. 33 represents another experiment which belongs to the laws of resisting force. A sixpence is placed on a table covered with a cloth or napkin. It is covered with a glass, turned over so that its brim rests on two penny pieces. The problem to be solved is how to extract the sixpence from underneath the glass without touching it, or slipping anything beneath it. To do this it is necessary to scratch the cloth with the nail of the forefinger; the elasticity of the material communicates the movement to the sixpence, which slowly moves in the direction of the finger, until it finally comes out completely from beneath the glass.

We may give another experiment concerning Inertia. Take a strip of paper, and upon it place a coin, on a marble chimney-piece, as in the illustration. If, holding the paper in the left hand, you strike it rapidly and forcibly, you will be enabled to draw away the paper without causing the coin (say a five-shilling-piece) to fall down (fig. 34).

It is not impossible to draw away a napkin laid as a tablecloth for one person’s dinner, without disturbing the various articles laid upon it. A quick motion is all that is necessary, keeping the napkin tightly extended by the hands at the same time. This latter experiment, however, is not recommended to boys home for the holidays, as they may unwillingly practise a feat analogous to that executed by Humpty-Dumpty, and find equal difficulty to match the pieces.

Fig. 34.—Drawing a slip of paper from beneath a coin.

We will now examine the term Motion. A body is said to be in motion when it changes its position in relation to surrounding objects. To perceive motion the surrounding objects must be relatively at rest, for if they all hurried along at the same rate no motion would be perceptible. This is evident, for when we stand still trees and houses appear stationary, as do we ourselves, but we know we all are rushing round with the earth, though our relative positions are unchanged. Hence there is no absolute rest.

What are the causes of motion?—Gravity is one. The influence of heat, which is itself caused by the motion of atoms, the effects of electricity, etc., and finally, the power of force in men or animals—any of these causes will produce motion. But a body at rest cannot put itself in motion, nor can a body in motion stop itself, or change its condition of motion.

But you may say a body will stop itself. Your ball on the ground, or even upon ice, will eventually come to a stop. We fire a bullet, and it will stop in time. We reply it does not stop of itself, The resistance of the Air and Friction tend to bring the body in motion to a state of rest. In the case of a bullet gravity brings it down.

There is no need to insist upon the resistance offered by the air even when it is not rushing violently past to fill up a vacuum beyond us, and called a breeze, or high wind. But we may say something of Friction.

Friction is derived from the Latin frico, to rub, and expresses the resistance to motion which arises from uneven surfaces. It is a passive resistance, and depends upon the force which keeps the bodies together. Thus a train running upon a smooth iron rail would never be able to proceed but for friction, which gives the necessary purchase or grip to the wheel and rail in contact.

No surface is perfectly smooth, for we must push a body upon the smoothest surface we possess. Friction tends to resist motion always, and is the cause of a great loss of power in mechanics, though it is employed to stop motion by certain appliances, such as “brakes” and “drags,” for sliding friction is greater than rolling friction. But without friction most structures would fall to pieces, and all forward motion would cease. So though it is an inconvenient force to overcome, we could not do without it.

If a body is set in motion, we see that the tendency of it is to go on for ever. Such, indeed, is the case with the stars; but so long as we are within the influence of the earth’s attraction, we cannot expect such a result. We know now what motion is; we must also, to understand it perfectly, consider its direction and its velocity.

The line which indicates the way from the starting point to the end is the direction of the object in motion, and the rate it moves at its velocity. The latter is calculated at so many miles an hour, as a train; or so many feet in a second if the object be a shot, or other very rapidly-moving body. In equal velocity the same distance is traversed in the same time; and so if a train run a mile in a minute, we know it will travel sixty miles in an hour, and is therefore during that minute going at the rate of sixty miles an hour. We have already spoken of the velocity of a stone falling from a cliff as sixteen feet in a second, and a stone thrown into the air to rise sixteen feet will be a second in going up, and a second in descending. But the velocity will be accelerated in the descent after the first second of time, and retarded in the upward cast by gravity. So we have two terms—accelerated and retarded velocity—used to express an increased or decreased force of attraction.

Perpetual motion has often been sought, but never discovered, nor will it ever be till the elixir of life has been found. It is quite impossible to construct any machine that will work without friction; if any work be done energy will be expended and transformed into other energy, so the total must be diminished by so much as was employed to transform the remainder. No body can give unlimited work, therefore the perpetual motion theory is untenable and impossible.

Fig. 35.—The pendulum.

The pendulum is considered the nearest approach to perpetual motion. This is so well known that no description is needed, but we may say a few words concerning it. By the diagram, we see that if we lift the ball to b, and let it fall, it will descend to l, and pass it to a opposite, nearly as far from l as b is from it. So the oscillations will continue, each beat being less and less, till rest is reached by the action of gravity (page 23). Were it not for friction and the pressure of the air, the oscillations would continue for ever; as it is, it declines by shorter swings till it remains in equilibrium.

The seconds’ pendulum oscillates sixty times an hour, and must be of a certain length in certain places. In London it is 39·1393 inches, and furnishes a certain standard of length, and by an Act of Parliament the yard is divided into 36 parts, and 39·1393 such parts make the seconds’ pendulum in the latitude of London (in vacuo) in a temperature of 62°.

Fig. 36.—Centrifugal Force.

But the same pendulum will not perform the same number of oscillations in one minute in all parts of the globe. At the equator they will be less, and at the pole more. Thus it was discovered that, as the movements of the pendulum are dependent upon the force of gravity, and as this force decreases the farther we get from the centre of the earth, the equator must be farther from the earth’s centre than the poles, and therefore the poles must be depressed. The decline of the pendulum at the equator is also, in a measure, due to Centrifugal Force.

Centrifugal Force, which means “flying from the centre,” is the force which causes an object to describe a circle with uniform velocity, and fly away from the centre; the force that counteracts it is called the centripetal force. A very simple experiment will illustrate it.

Fig. 37.—Another illustration of centrifugal force.

To represent its action, we shall have recourse to an ordinary glass tumbler placed on a round piece of cardboard, held firmly in place by cords. Some water is poured in the glass, and we then show that it can be swung to and fro and round without the water being spilt, even when the glass is upside down (fig. 36).

Another experiment on the same subject is as shown in the above illustration, by which a napkin ring can be kept in revolution around the forefinger, and by a continued force the ring may be even held suspended at the tip of the finger, apparently in the air, without support (fig. 37).