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First Law of Motion-Inertia.

"The so-called inertia of Rest."

149. When we put our hand to lift a heavy weight or to turn a heavy fly-wheel, we seem to experience a resistance or unwillingness of the mass to be set in motion, a sort of stubbornness which it takes some time to overcome, as if by our efforts we had to persuade the mass to move. This is, however, but a metaphorical and illusory inference from our own feelings.

We have already explained that the measure of any force is dependent on the mass or quantity of matter moved and the velocity conjointly; it follows that, when a body of smaller mass imparts its force to a larger body, the latter will not move off with so great velocity as the former; and before it can acquire its speed, it must receive a succession of impulses from the small body. Now, it is this change of velocity in the transference of force that seems to countenance the idea that matter at rest offers a resistance to be set in motion. When we move a fifty-six pound weight, the velocity generated will be very much less than in the case where the hand alone is moved with the same effort; and the seeming unwillingness of the mass to move is but an erroneous mental inference from this fact.

150. Such is the proper explanation in the following examples, usually given to illustrate the stubbornness or inertness of matter:

The light wind blowing, even with a high velocity, on the newlyspread sails of a ship, will not impart at once a like speed to the heavy vessel. A continuous blowing, that is, a constant succession of such air-impulses, is needed to give it swift motion, or, as it is commonly put, before the inertia of the vessel is overcome.

The starting of a long railway train, by the powerful motion of the comparatively diminutive piston of the engine, exemplifies the same principle.

Horses starting a carriage expend repeated efforts before its motion is equal to what theirs would be with those efforts, if detached from this addition to their mass.

A man lying down and receiving the blow of a forge-hammer on his chest would be instantly killed by the sudden inward pressure; but if he can suffer an anvil to be laid on his chest the blow may then be given with impunity. The blow diffused through the enorinous mass gives a velocity so trifling as to be uninjurious.

Examples of Inertia of Rest.

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151. Where one moving body meets another not directly, but very obliquely, so that their surfaces just graze, in general only a small quantity of the motion of the first will thus be communicated to the second; and it requires long continued rubbing, or friction, that the whole may be transferred. The rubbing surfaces are in no case perfectly smooth, and we may suppose the motion communicated by the impacts of minute projections or protuberances on cach surface; and, of course, the rougher the surfaces, or the greater the friction between them, the more rapid will be this communication of the motion, because the nearer it will approach direct impact of the masses.

Now many of the examples usually given of inertia merely exemplify the fact that the imparting of force by friction, is necessarily more tardy in producing velocity than by direct communication.

Of this class are the following examples :

If a shilling be laid on a smooth card balanced on the tip of the finger, a sharp blow against the edge of the card will cause it to dart off, leaving the shilling apparently undisturbed on the finger; the momentary friction was not sufficient to give the mass of metal any perceptible motion.

So a person standing carelessly at the stern of a boat, or on the step of an omnibus paying his fare, may, by a sudden start of the vehicle, be left behind.

An awkward rider may be left behind by his horse starting off suddenly; or he may be thrown to one side by the horse starting to the other.

A person in a carriage has his head thrown against the cushions when the carriage is first put in motion: because he is not glued rigidly to it, so as to take its velocity at once.

In those dreadful railway collisions which are but too common in our day, a passenger facing the engine will be dashed on his face and receive a blow on the forehead, while one sitting with his back towards the engine will receive a no less deadly blow as from a hammer on the back of the head.

152. If the parts of a body be stiffly connected, any force imparted to the body will produce a similar velocity of all the parts. But if the connexion between the parts be elastic, or not rigid, one part may be moved, and the force communicated through the elastic connection will not be sufficient to give the other part equal velocity, the two parts will not move together, but one part will seem to wish to remain behind the other.

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Examples of so-called Inertia.

Thus, if a glass of wine be suddenly pushed forward on a table, the friction between the liquid and the glass is not sufficient to allow communication of equal velocity to the liquid, and the wine is left behind on the cloth.

A servant carrying away a tray of glasses or china quickly may let some drop from this cause.

A broad basin filled with liquid, such as a tub of water or a plate of soup, must be very gently carried, so that the friction between the liquid and vessel get time to generate a velocity in the liquid equal to that of the basin.

The operation of beating a carpet, or of a sheep shaking off the snow or wet is similarly explained.

A weight suspended by a spring on ship board is seen vibrating up and down as the ship pitches with the waves. It seems to fall as the ship rises, and rise as the ship falls; but the motion is mostly in the ship, that which is transmitted through the elastic support not being sufficient to give the heavy mass a sensible velocity.

A heavy weight so supported and connected with a pump-rod has been caused to work the pump.

Like the weight last mentioned, the mercury of a common barometer on ship-board appears to be constantly rising and falling in the tube as the ship pitches; and until the important improvement was made of narrowing a part of the tube to prevent this, the mercurial barometer was useless at sea. The explanation is that the tube rises and falls with the ship from being affixed to it; but as it rises, the friction between the glass and mercury is not sufficient to communicate to the heavy liquid a motion equaily quick with that of the tube; and we, rising with the vessel, and unconscious of our real velocity, interpret the insensible velocity of the mercury as a falling down instead of a slight rising, which is actually the case. So when the ship pitches suddenly down, its motion is more rapid than that which gravity imparts to the mercury, and accordingly the mercury seems to rise in this case.

The well-known mode of supporting a ship's compass-box on gimbals-which may be called a sort of universal pivoting-is another illustration of the same natural law. An interesting and useful one also is Bessemer's invention, which is intended to alleviate the sorrows of the sea-passage from Dover to Calais, and which consists in suspending a whole cabin or saloon in the middle of a steamer, so as to move about a longitudinal axis parallel to the keel. As there is no part of the vessel fixed, there would of course

Motion as natural as Rest.

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be more or less oscillation set up in the saloon by the rolling of the ship; but, by an ingenious application of hydraulic machinery, Bessemer places the damping of these oscillations under the control of one man, whose duty it is to keep the floor of the saloon horizontal, being guided by a spirit level in front of him, just as the helmsman is guided by the compass.

A heavy mass, balanced by springs within a frame and carrying a pencil, may by its comparatively insignificant motion be caused to trace, on a sheet of paper rigidly fixed to the frame, all quick motions of the latter. In this way the tremors and heavings of the ground in earthquakes can be recorded, or the severe jolts of railway carriages can be made to indicate defects in the road needing repair.

153. The second statement in this First Law of Motion is, that a body once in motion will continue to move uniformly, if no external force interfere. In other words, any change in the velocity or the quantity of momentum of a moving body is due to the action of some external matter or force.

Prevent this, isolate the moving body from all other matter and force, set it in motion, and in eternal, perpetual motion it will continue so long as it traverses absolutely empty space. Here, again, we seem to be contradicted by the experience of every-day life, and the common prejudice founded on it, that moving matter has somehow a tendency to rest, and that motion is an unnatural and more or less temporary condition. All artificial motions, and all natural motions around us on the globe tend, sooner or later, to cease: we may make a clock to go for a week, or a month, or even a year, and we admire its ceaseless activity whether we wake or sleep. But stop it ultimately must; rest is the fate which there is no escaping. The grand mechanical impossibility is a perpetual motion. And why? Simply because we cannot annihilate the action of all external forces. Do what we may, we can never eliminate the interference of friction alone; until we can poise a ball in utter vacuity and produce a perfect copy of the planetary and stellar masses we must be content to bear the doom of our condition-the gradual leakage of all force by its diffusion through surrounding matter. Friction of the air and of other matter is the great draining force which tends to stop all motions on the surface of the earth; but the more that we lessen this, the nearer we come to realize the truth of this second part of the

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Persistence of Free Motion.

law of motion-that motion is as natural and as persistent as rest if left absolutely free.

154. The illusion of this idea of preference for rest will appear when we consider the following illustrations :

A top or a gyroscope may keep up its motion for ten, twenty, thirty, minutes or more: but sooner or later the whirling ceases, simply because there is a gradual transference of its momentum to the surrounding air-where the motion ceases to appear-and to the molecules near the point of support in the shape of an invisibly minute heat-quiver. If we set the top spinning inside a vessel from which we can extract the air, it will go on whirling for hours, because one of these avenues of expenditure is closed. From this we are justified in concluding that could we put it in motion in an absolutely empty space, and free from contact with any other matter, it would never stop.

Practically, it is impossible to do this. There is always more or less transference, diffusion, or apparent loss of visible motion at the points of support of any contrivance, however fine these may be made; everything is, moreover, bathed in an ocean of air particles, and these take up part of the motion and lead to its gradual disappearance.

A pendulum, or a leaden ball hung by a fine silk thread, will swing only for a few minutes in air before this friction destroys its motion. In a vacuum it may vibrate for nearly a whole day, because the only source of leakage is at the point of support.

So a ball rolled on level grass soon stops; rolled over a smooth floor it goes on longer, and on smooth ice much longer still, the loss by friction being then reduced to a minimum.

155. It is only in the celestial spaces, however, that we see

motions completely freed from the interference of air or
other matter, and there they seem eternal.

Had the human eye, unassisted, been able to descry the four beautiful moons of Jupiter wheeling around him for these thousands of years with such an unabated regularity that they now form to the astronomer an unerring time-piece in the sky, the prejudice that motion is always tending to rest would never have arisen.

Science has proved that the velocity of our globe, in its present orbit, was thousands of years ago exactly as it is at the present day; and that the length of the day has not varied by so much as a second. And this is simply the result of the perfect vacuity of

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