Now the measure of the elasticity of a body is proportionate to the velocity of the wave-motion which it can transmit. A good illustration of the transmission of wave-motion may be shown with a number of ivory bagatelle or billiard balls. If eight or more of these be put in a row, all touching each other, and a single ball be placed about an inch or so away from the others in a straight line with them, then when the single ball is struck with a cue against the other eight, the motion of the single ball is transmitted by each one of the eight successively with such rapidity, that the end ball would be set in motion in a quicker time than a single ball would take to reach the end ball, if it had been free to move along without encountering any opposition.
It is a fact capable of demonstration, that the smaller the particle of matter, the greater will be its vibratory motion. Thus the particles of air are very, very small, and consequently air is found to be very elastic, and allows sound to be transmitted through it with comparatively great velocity, some sounds travelling at the rate of over 1000 feet per second.
A most important factor in determining the propagation of any wave-motion, through a gas or solid, is the relationship of the elasticity of the gas or solid to its density. Suffice to say, that the velocity of any wave-motion is determined by the relation of the elasticity to the density. For example, sound, which is a wave-motion of the air, can not only be transmitted through gaseous bodies as air, but also through liquids and solids. Sound travels faster through solids than through liquids, and faster through liquids than through gases. In liquids, the relation of the elasticity to density is greater than in air, and in solids the relation is greater still. Therefore sound travels much faster in liquids than in gases, and faster in solids than in liquids.
This is the reason why a train can be heard coming if the ear is put to the railway-line, when no indication of its approach is given to the ear by the atmosphere. Some examples of the velocities of sound through different substances are as follows--
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Art. 40. Matter possesses Inertia.--Inertia is that property of matter, by which matter cannot of itself alter, or change its state of motion, or of rest.
Newton's first law of motion states that a body at rest remains at rest until some force or motion acts upon it. If a stone be dropped from a balloon, the stone does not fall because of any property which it possesses, but because the force of gravity acts upon it. If it were possible to eliminate this force of gravity, then if there were no other force which could act upon the stone, it would remain suspended in space.
The inertia of a body is equal to the mass of that body, or the amount of matter in the body as measured by gravity, so that if a body is halved, its inertia will be halved also, and if doubled, its inertia will be doubled also. As the inertia of matter opposes all kinds of motion, the amount of force required to overcome the inertia of a body is proportionate to its mass. So that if the mass of a body is doubled, then twice the force would be required to move it, while if the body were halved, half the force would suffice to do it.
Inertia is possessed quite as much by a moving body as a body at rest. The definition given points this out, as it states that matter cannot of itself change its state of motion. If a body therefore is in motion, it requires a certain amount of resistance to bring the body to a state of rest, or the loss of an equal amount of energy, by friction or otherwise, equal to the quantity which it absorbed in order for it to be set in motion.
We get numerous examples of this property of the inertia of bodies in our daily experience. Many of the accidents that befall people in various ways are due to this property of the inertia of matter. A cyclist is riding a machine down-hill, and loses control over his machine, with the result that he runs into a wall, and is killed. Now what has happened? The cyclist has participated in the motion of the machine, with the result that when the machine has been suddenly stopped, the body has been thrown forward owing to the momentum it had acquired.