The Quantity of Matter is the measure of the same, arising from its density and bulk conjunctly.
THUS AIR of a double density, in a double space, is quadruple in quantity; in a triple space, sextuple in quantity. The same thing is to be understood of snow, and fine dust or powders, that are condensed by compression or liquefaction; and of all bodies that are by any causes whatever differently condensed. I have no regard in this place to a medium, if any such there is, that freely pervades the interstices between the parts of bodies. It is this quantity that I mean hereafter everywhere under the name of Body or Mass. And the same is known by the weight of each body, for it is proportional to the weight, as I have found by experiments on pendulums, very accurately made, which shall be shewn hereafter.
The Quantity of Motion is the measure of the same, arising from the velocity and quantity of matter conjuctly.
The motion of the whole is the Sum of the motions of all the parts; and therefore in a body double in quantity, with equal velocity, the motion is double; with twice the velocity, it is quadruple.
The Vis Insita, or Innate Force of Matter, is a power of resisting, by which every body, as much as in it lies, endeavours to persevere in its present state, whether it be of rest, or of moving uniformly forwards in a right line.
This force is ever proportional to the body whose force it is; and differs nothing from the inactivity of the Mass, but in our manner of conceiving it. A body, from the inactivity of matter, is not without difficulty put out of its state of rest or motion. Upon which account, this Vis insita may, by a most significant name, be called Vis inertiæ or Force of Inactivity. But a body exerts this force only, when another force, impress'd upon it, endeavours to change its condition; and the exercise of this force may be considered both as resistance and impulse; it is resistance in so far as the body, for maintaining its present state withstands the force impressed; it is impulse in so far as the body, by not easily giving way to the impress'd force of another, endeavours to change the state of that other. Resistance is usually ascrib'd to bodies at rest, and impulse to those in motion: But motion and rest, as commonly conceived, are only relatively distinguished; nor are those bodies always truly at rest, which commonly are taken to be so.
An impress'd force is an action exerted upon a body, in order to change its state, either of rest, or of moving uniformly forward in a right line.
This force consists in the action only; and remains no longer in the body, when the action is over. For a body maintains every new state it acquires, by its Vis Inertiæ only. Impress'd forces are of different origines; as from percussion, from pressure, from centripetal force.
- Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed thereon.
Projectiles persevere in their motions, so far as they are not retarded by the resistance of the air, or impelled downwards by the force of gravity. A top, whose parts by their cohesion are perpetually drawn aside from rectilinear motions, does not cease its rotation, otherwise than as it is retarded by the air. The greater bodies of the planets and comets, meeting with less resistance in more free spaces, preserve their motions both progressive and circular for a much longer time.
- The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed.
If any force generates a motion, a double force will generate double the motion, a triple force triple the motion, whether that force be impressed altogether and at once, or gradually and successively. And this motion (being always directed the same way with the generating force), if the body moved before, is added to or subducted from the former motion, according as they directly conspire with or are directly contrary to each other; or obliquely joined, when they are oblique, so as to produce a new motion compounded from the determination of both.
- To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.
Whatever draws or presses another is as much drawn or pressed by that other. If you press a stone with your finger, the finger is also pressed by the stone. If a horse draws a stone tied to a rope, the horse (if I may so say) will be equally drawn back towards the stone: for the distended rope, by the same endeavour to relax or unbend itself, will draw the horse as much towards the stone, as it does the stone towards the horse, and will obstruct the progress of the one as much as it advances that of the other. If a body impinge upon another, and by its force change the motion of the other, that body also (because of the equality of the mutual pressure) will undergo an equal change, in its own motion, towards the contrary part. The changes made by these actions are equal, not in the velocities but in the motions of bodies; that is to say, if the bodies are not hindered by any other impediments. For, because the motions are equally changed, the changes of the velocities made towards contrary parts are reciprocally proportional to the bodies. This law takes place also in attractions, as will be proved in the next scholium.
Surely there is no technology that has changed our lives more than the electricity that comes into our homes on a wire. Lighting, cooling, heating, ironing, cooking, and entertainment at the flick of a switch. Cheap, silent, invisible energy at our beck and call, flowing though a wire.
One of my scientific heroes is Michael Faraday -- gentle, brilliant, infused with wonder. No one did more to wrest electricity from the gods and make it do our bidding than he. For most people of his time, electricity was a curious novelty, a parlor game. Faraday understood it another way:
Electricity is often called wonderful, beautiful; but…The beauty of electricity or of any other force is not that the power is mysterious, and unexpected, touching every sense unawares in turn, but that it is under law, and that the taught intellect can govern it largely.
There you have it, as perfect a statement of the scientific spirit as you are likely to find.
Everyone who flicks a switch to turn on the light and at the same time espouses a belief in miracles is embracing a kind of cognitive dissonance. The lightning bolt that jags across the sky is not the whim of a willful Zeus -- or as we might say, an act of God -- it is lawful. Behind all of the apparent randomness of nature, law prevails. And the taught intellect can govern it.
1. Gravity. This is the weakest of the four, but it is a long-range force and acts on everything in the universe as an attraction. This means that for large bodies the gravitational forces all add up and can dominate over all other forces.
2. Electromagnetism. This is also long-range and is much stronger than gravity, but it acts only on particles with an electric charge. being repulsive between charges of the same sign and attractive between charges of the opposite sign. This means the electric forces between large bodies cancel each other out, but on the scales of atoms and molecules they dominate. Electromagnetic forces are responsible for all of chemistry and biology.
3. Weak nuclear force. This causes radioactivity and plays a vital role in the formation of the elements in stars and the early universe. We don't, however, come into contact with this force in our everyday lives.
4. Strong nuclear force. This force holds together the protons and neutrons inside the nucleus of an atom. It also holds together the protons and neutrons themselves, which is necessary because they are made of still timer particles, the quarks we mentioned in Chapter 3. The strong force is the energy source for the sun i and nuclear power, but, as with the weak force, we don't have direct contact with it.
Our modern understanding of the term "law of nature" is an issue philosophers argue at length, and it is a more subde question than one may at first think. For example, the philosopher John W. Carroll compared the statement "All gold spheres are less than a mile in diameter" to a statement like "All uranium-23 spheres are less than a mile in diameter." Our observations of the world tell us that there are no gold spheres larger than a mile wide, and we can be pretty confident there never will be. Still, we have no reason to believe that there couldn't be one, and so the statement is not considered a law. On the other hand, the statement "All uranium-235 spheres are less than a mile in diameter" could be thought of as a law of nature because, according to what we know about nuclear physics, once a sphere of uranium-235 grew to a diameter greater than about six inches, it would demolish itself in a nuclear explosion. Hence we can be sure that suet spheres do not exist. (Nor would it be a good idea to try to make one!) This distinction matters because it illustrates that not all generalizations we observe can be thought of as laws of nature. and that most laws of nature exist as part of a larger, interconnected system of laws.