Isaac Newton (1642-1727) was a great mathematician and a great
physicist, and he was probably the most incisive thinker ever known. He
chiefly established that natural phenomena generally follow
determinate mathematical laws in demonstrating consistent laws of
motion, of gravity and of other phenomena. While apparently backing Gilbertian attraction physics, he produced his 'black
box' theory of science as allowing only proving how things happen but not necessarily
allowing proving alternative theories of why things happen like Einstein's theory.
Of course the classic 'black-box' was a thing that hides stuff from view and so prevents explanation, but perversely a 'black-box' has now become a thing
that records stuff for later viewing and so provides explanation and which would be better called a 'white-box' !?
Newton may have seen his blackbox physics as simply a more rigorous logical requirement of experimental science, from William Gilbert's earlier requirement that science could not go beyond what can be deduced directly from experience and experiment. Though it was supported by a few other physicists, and by George Berkeley in his 1721 'De Motu', Newton's black-box physics has been very wrongly either just ignored or 're-interpreted' by many including Einstein as being a Cartesian 'dead-matter plus ether or forcefield' push why-physics. Newton's main works were his Latin 1687 Philosophiae Naturalis Principia Mathematica, and The Opticks published in English in 1704 and in Latin in 1706.
Though Newton had earlier favoured Descartes-style
mechanical push physics theory, his major theory work seems to have involved
combining laws of force and motion in mechanics with a Gilbert-like
attraction theory, to develop laws of gravitational orbital motion
around attracting 'centres of force'. This change of view may have been chiefly come from Robert Hooke suggesting that planet orbitings might be
due to attraction forces decreasing with distance compounded with inertial forces of planets momentum, though he had considered attraction physics theory earlier. Newton confirmed that the maths of planet motion did fit attraction physics and saw gravity as governing the
motions of the celestial bodies as well as of apples falling from
trees. But Newton publicly supported a 'blackbox physics' or 'theoryless physics', and his main work used both the 'force' terminology perhaps more readily
associated with Galileo-Descartes mechanics AND the 'attraction'
terminology associated often with William Gilbert attraction
theory, and allowed that gravity might be due to unseen signals
acting across empty space in line with Gilbert's physics OR might be
caused by the impact force of unseen ether particles or fluid in line
with Descartes' physics. But privately Newton certainly seems to have judged the evidence to favour Gilbert action-at-distance signal physics
so Newtonian physics cannot be really understood unless you have first studied William Gilbert's 'De Magnete' or 'On The Magnet'.
Hence, in the Principia final Scholium to Book 1 Section 11(or X1), after showing that planet orbits can be explained by some centripetal force directed towards the sun, Newton concludes that the existence of gravity as a property of bodies can be deduced from the proven existence of magnetism as a property of bodies ;
"These propositions naturally lead us to the analogy there is between centripetal forces, and the central bodies to which those forces used to be directed ; for it is reasonable to suppose that forces that are directed to bodies should depend on the nature and quantity of those bodies, as we see they do in magnetical experiments."
Yet also in this scholium Newton states that he is not committing to any particular manner of operation of 'at-a-distance' forces or of 'contact' forces.
"I here use the word attraction in general for any endeavour, of what kind soever, made by bodies to approach each other ; whether (as Gilbert) that endeavour arise from the action of the bodies themselves as tending mutually to or agitating each other by spirits emitted ; or whether (as Descartes etc) it arises from the action of the aether or of the air or of any medium whatsoever whether corporeal or incorporeal any how impelling bodies placed therin towards each other. In the same sense I use the word impulse, not defining in this treatise the species or physical qualities of forces but investigating the quantities and mathematical proportions of them"
Clearly to Newton bodies moved, but experiment could not definitely decide if they were actually being pushed by others or moving themselves - there is no evidence to decide between dead matter and active matter or between 'A moves B', and 'B moves itself in response to A'. But in fact Newton did get actual attraction physics to fit the evidence and could not get the current Descartes push-physics to fit the evidence. (A modern science-english translation of Newton here maybe would not be 'spirits emitted' but 'incorporeal emissions', 'energy emissions', 'incorporeal agitating emissions' or 'force signal emissions'? But in the General Scholium ending his Principia he does indicate a maybe less scientific view on 'spirits' within bodies reacting to such emissions?)
Of course Newton's conclusion that his evidence strongly supported attraction physics or blackbox physics against mainstream Cartesian Atomist push physics was (stupidly) not accepted by most physicists (who Newton noted in Principia's introduction to Book 3, had "prejudices to which they had been many years accustomed"), and it was maybe too difficult for Einstein or anyone else to address. But Newton saw his laws of science as correctly predicting natural events without needing to know why things happened, in the manner of 'black box' behaviour laws that relate only inputs or stimuli to outputs or responses without considering any mechanisms connecting them. Newton considered hypotheses regarding currently unseens as being matters of philosophy or logic and not science, and not currently provable or disprovable by science. Newton concluded that though he had disproved substantial elements of Galileo-Descartes mechanical physics, like ether vortex motion gravity and motion tides, some future modification of a mechanical ether theory might correctly explain gravity, magnetism, electricity and light. But Newton himself seemingly preferred to use Gilbert-style attraction theory in thinking about physics, which he also seemingly thought might more likely correctly explain gravity and some or all other forces.
Newton's considerations on Descartes push-physics as against Gilbert response attraction physics is maybe best put in his Principia Book 3 Rule 3. Here he first shows how we can reason that matter has solidity and exclusive-space-occupancy, then how "we must universally allow that all bodies whatsoever are endowed with a principle of mutual gravitation." Then he concludes that the argument is stronger for the universal gravitation of all bodies than for their impenetrability. But in finding that Gilbert-like physics was somewhat more likely the true option, Newton concluded that the evidence did not exist to decide between the two theories and might well never exist, continuing with "In bodies we see only their figures and colours, we hear only the sounds, we touch only their outward surfaces, we smell only the smells, and taste the savours : but their inward substances are not to be known either by our senses or by any reflex act of our minds" - Newton could see no evidence for Descartes 'certain knowledge'. He basically concluded that the evidence did not exist to decide between taking 'mass' as the measure of the size and pushability of bodies or as the measure of bodies' ability to produce and respond to gravity signals - though he seems clearly to have privately favoured the latter. But of some interest Newton's scholium words above also allow that INCORPOREAL bodies or media might either excite responses from matter or might somehow push matter though lacking any push property in Cartesian physics.
While many physicists see explaining 'at-a-distance forces' like magnetism and gravity as more problematic than explaining 'contact forces' like collision, Gilbert and Newton saw explaining both types of forces as being equally problematic. Unlike Descartes they saw trusting logical deduction or human senses to always give direct 'certain knowledge' as being unscientific - to them 'contact' could be 'small-separation' and everything really needs rigorous experimental proof, even if at that time or any time such is not possible. And mostly experimental science should be proving what is more probably true, rather than what is true. Proofs of substantial distances may often be more reliable than proofs of zero distances which commonly require the unproven assumption that what is not visibly large must be zero, doubtful even before microscopes. There is no good scientific proof of the actual existence of any kind of 'contact push force' though such forces may seem to exist. And a majority of scientists insisting on push-physics when the facts demonstrated by Gilbert and Newton made it very doubtful was 'science prejudice' prevailing against Newton sense - as it was still doing so in 2015 with Brian Cox talking nonsense about Newton on TV in England as supposed science.
Newton spent a great amount of time doing experiments in optics, magnetism and materials chemistry though he published more about physics theory. He was annoyed that his early groundbreaking experiments were misunderstood and misrepresented by many of his peers but did not let that stop him doing further experiments but did put him off further publishing for a long time. He also did useful work on sound, and produced a theory of fluids that solved problems of fluids in movement and of motion through fluids. This he applied to Descartes' supposed unseen universal fluid ether, in which many physicists came to believe, but Newton disproved substantial aspects of that and he never conceded any kind of mediating ethers nor indeed signals as proven entities though granting that action-at-a-distance needed some kind of mediation. He did in his 'Opticks' and elsewhere use both ether explanation and attraction explanation to help clarify his new physics ideas, especially for physicists who supported either one of such explanations and their 'unseens'. Many at the time saw Newton as developing Gilbert's theory which supporters of Descartes' Cartesian push-physics had made very unpopular by name-calling only, but one of Newton's great originalities was in his seeing particular explanations as unnecessary to science and seeing hypotheses on unseens as being unscientific - and being the first clear proponent of a blackbox science simply predicting everything. Copernicus, Galileo and others had earlier done some black-box science, but excluded explanation only either as being more politic or as to be perhaps done later. The substantial unpublished writings that Newton left after his death showed that Newton had major concerns about which he was unwilling to publish anything in his lifetime, like his major concern with Chemistry, seemingly being fearful of peer pressure prejudice.
Mathematics was also advanced by Newton's work on calculus, which many of his peers falsely claimed was stolen from Leibniz though there was evidence of the opposite. But the much bigger stealing from Newton was undoubtedly Cartesian physicists falsely claiming that his Principia physics maths supported their Cartesian physics rather than attraction physics or blackbox physics. While Newton's science was presentationally mathematical and distinctly in the style of Euclid, Newton always required that experimental facts must be decisive in science and not mere logical deduction or mathematics alone. Much of Newton's time was devoted to experiments, and of course Newton's published physics mathematics was, like Gilbert's and most early mathematics, presented geometrically rather than algebraically.
Newton was the chief proponent of defined mathematical behaviour laws with undefined-explanation 'black-box science', maybe chiefly because he could see no way to certainly decide or prove between the alternative Gilbert and Descartes physics explanations ('Newton's Dilemma') or between equivalent alternative explanations of light. If different theories could fit the same mathematics then maybe they were either really the same theory or were compatible image theories and descriptions that only appeared different. Newton did convince a few other scientists of his time into favouring Black Box physics that could predict everything without relying on explanations, as being the best physics possible as long as there were no proven physics theories without unseens. But explanation-theory retained its popularity among scientists and was even credited to Newton ironically. Black-box theory was maybe fine while nature was seen as being relatively simple, but it perhaps looked less intelligible when nature became seen as being more complex - so it could be argued that defined explanation is then needed to help make a theory more understandable ? Or maybe some correct science theory cannot be understandable to many anyway ? Of course a science theory cannot be only a bare mathematics with no physical meaning, but it can be a mathematics whose physical meaning is not fully uniquely defined.
Newton did not well explain his two explanation-physics options, nor refer people to Descartes and Gilbert for such explanations, but most of his peers believing him to support Gilbert's physics did not want Newton saying so. Newton undoubtedly knew how badly Gilbert's earlier attraction physics theory had been treated, and correctly expected that a theory substantially based on it would likely be equally badly received especially if it referred to Gilbert. Of course Newton giving little explanation allowed many to misunderstand or misrepresent Newton's physics as still holds today. It certainly appears that Newton knew of Gilbert's main experiments on magnetism and static electricity, and that he himself replicated some of those experiments. But he seems to not have had any substantial teaching of Gilbert and to not have done any substantial study of Gilbert. Unusually at least two of Newton's acquaintances, including Robert Hooke, supported Gilbert's attraction physics theory to at least some extent rather than the more popular Cartesian push physics theory - but it is not clear to what extent they influenced eachother in that respect. Hooke did put some attraction physics to Newton, but it is probable that had Newton studied Gilbert properly then he could have developed his own physics further than he did ? (though neither Hooke or Newton acknowledged Gilbert, maybe someone still has evidence connecting them to Gilbert or Gilbertian ideas ?)
Newton did try publishing one short paper on a part of his optics work submitted in 1672 to the Royal Society. This first paper was a small correct non-theory technical paper on colours, colour aberration and Newton's new reflecting telescope - fully proving all that it said. But amazingly the eminent physicist peer Robert Hooke immediately tried to stop the Royal Society publishing this first paper of Newton, and himself published a ridiculous factually-wrong criticism of it that was widely backed. In reply to Newton rightly defending his paper in 1675 an angry Robert Hooke threatened to form his own Royal Society, yet it was widely said that Newton was unreasonable! www.newtonproject.sussex.ac.uk/view/texts/normalized/NATP00006 Then in 1684 Gottfried Leibniz after visiting Newton and seeing some of his maths began publishing some of Newton's key mathematics as his own, but by 1690 many were claiming that Newton had stolen Leibniz maths. Newton decided against publishing further papers, and though he held a higher opinion of some earlier thinkers like Euclid, he was very wary of putting his own ideas to most of his peers. With a few minor mostly anonymous exceptions and private letters to a few friends, Newton waited until he could publish his science himself complete in book form - his Principia in 1687 and his Opticks in 1704. And when they were dismissed without disproofs by largely Descartes-supporter peers, Newton resigned his Cambridge professorship to finish with science and found himself a new job as head of the British Mint. He stopped his science theory work and his science experiments and devoted his energies to his new job instead to his death in 1727, though he did continue to basically defend his physics through the Royal Society. He had maybe decided that science choose to proceed by lying like politics and religion both of which he also had some involvement with ? And maybe that he had been too open in publishing his science relatively early ?
As with Gilbert earlier, Newton's attraction physics was rubbished without disproof as being anthropomorphic, including silly claims that it required all matter to have eyes, minds and legs - ridiculous claims that themselves involve anthropomorphic thinking. Gravity being simple can clearly need only the simplest response, and the relative nature of attraction theory really gave it more scientific power. And simple single-cell creatures like Amoeba can show various responses to light and other things though having no brain, as can a TV to a remote. But Newton's black-box theory was soon simply ignored as though it did not exist.
To quote 'A Short Account of the History of Mathematics' (4th edition, 1908) by W. W. Rouse Ball, on Newton -
" His theory of colours and his deductions from his optical experiments were at first attacked with considerable vehemence. The correspondence which this entailed on Newton occupied nearly all his leisure in the years 1672 to 1675, and proved extremely distasteful to him. Writing on December 9, 1675, he says, `I was so persecuted with discussions arising out of my theory of light, that I blamed my own imprudence for parting with so substantial a blessing as my quiet to run after a shadow.' Again, on November 18, 1676, he observes, `I see I have made myself a slave to philosophy; but if I get rid of Mr.Linus's business, I will resolutely bid adieu to it eternally, excepting what I do for my private satisfaction, or leave to come out after me; for I see a man must either resolve to put out nothing new, or to become a slave to defend it.' "
A majority of Newton's peers were strong Descartes push-physics supporters who would not consider alternative theories, and especially would not consider the old enemy Gilbert attraction theory. They saw Newton as an anti-Descartes Gilbert theorist and believed that Newton's blackbox position was just a fraudulent cover to disguise his backing for the hated Gilbert theory. The minority of Newton's peers who would reasonably consider alternative theory ideas, mostly took Newton at face value as supporting blackbox theory and not attraction theory - and only few of them accepted black-box theory. Nobody other than Newton gave any real consideration to attraction theory, not even to attempt disproofs of it. And Newton himself produced no disproofs of it, only disproofs of parts of Descartes mechanical physics which suffered from more rigid requirements as do many other physics theories. Newton firmly held to his blackbox-science line dividing scientific knowledge from non-scientific knowledge - with religion and explanations of gravity and other forces being areas of great interest but outside science.
Newton privately seemingly tried unsuccessfully to develop his attraction physics effluvia/emitted-spirits theory by much experimenting on novel and new materials that only chemistry could produce, additional to his published experiments in magnetism and optics which latter led to his invention of the modern reflecting telescope. His private non-catholic religious ideas were seemingly much more specific and detailed than those of catholic Descartes, but his published attraction theory emitted-spirit ideas were maybe less developed than Gilbert's published effluvia ideas. And early chemistry then was still being demonised by being called alchemy even by most so-called scientists who should have known better when even in 1600 Gilbert had acknowledged chemists.
Newton like Gilbert became acclaimed as a great scientist, while the theories of both were rejected without disproof (much later Einstein did produce his 'disproof of Newton' which was eagerly accepted with nobody looking closely enough to see that the theory Einstein was actually disproving was Cartesian theory). The failure of Gilbert and Newton theory among physicists was not reflected among non-physicists, so that even today most people see their signal-attraction physics theory as correctly explaining magnetism and gravity. A caricature of part of Newton's physics theory became acclaimed somewhat slowly, with his real theory rejected with Gilbert's by the mob of scientific pigmy peers - and that process passed into physics history continues still now. Or maybe, being really generous, it could be said that the world was just not really ready to look at a physics that was not some simple mechanical push physics - and maybe the world is still not ready ?! Additional to fierce Catholic church opposition to early science, some opposition to early science (most often somehow directed at Newton) has been non-religious philosophical opposition as from some poets, artists and philosophers including Yeats and Goethe. Certainly it remains rare today to find an even half-reasonable scientific view of Gilbert-Newton physics outside of this website.
For comparison with other physics theories, Newton's three laws of
motion were ;
1. Every body will remain at rest, or in a uniform state of motion unless acted upon by a force.
2. When a force acts upon a body, it imparts an acceleration proportional to the force and inversely proportional to the mass of the body and in the direction of the force.
3. Every action has an equal and opposite reaction OR the mutual actions of two bodies on each other are equal and opposite.
Newton's view of 'a force acting' allowed of either some kind of
Descartes 'dead-matter' push action or Gilbert 'robot-matter'
signal attraction action from another body. It requires the existence of 1.a force from one body AND 2.a second body
acted upon by the force, with the actions of each being relative to each other.
He is maybe here not clear enough that his 'force' gives RELATIVE change of
motion, relative acceleration, rather than giving absolute change of motion and
that all motion is DIRECTIONAL or vectoral. While push-physics requires all forces to be directly
associated with an originating body, attraction physics allows some forces to
exist in signals separated from an originating body though allowing that the signals
themselves may be some kind of body. But for both, forces acting need BOTH
an originating body AND a body acted on and forces persist (as with collision,
spring and gas-pressure forces) for only as long as they are opposed. Also to Newton,
two equal and opposite forces produce equal and opposite accelerations giving no motion so
that force acceleration change acts primarily over time and not always also over distance or space. Newton did
with substantial success mathematise Gilbert attraction physics in these respects.
But the spacetime vectoral mathematics later developed by Minkowski would maybe
better suit it than being wasted on some merely geometrical physics.
Current mainstream physics commonly seems to say that the gravitational force between two bodies in Newton's physics is given by the formula F=G((m1m2)/r²)) which may imply that their mutual attraction is proportional to the product of their masses. But a pebble doubled in mass does not fall to the ground at double the acceleration, showing only an infinitesimal increase. Though now used as an approximation for mutual gravitation in terrestrial gravitation for masses tiny compared to Earth's mass, this mainstream 'mis-equation' is only about the hypothetical one-way effect of one gravitational mass on another inertial mass and might better reflect the physics, and Newton, as F=Gm1(m2/r²). And, as Newton required, the mutual attraction of two bodies is the simple sum of the gravities of each. This mere force addition may seem to some to undermine mutual causation, though push action-reaction mere force addition may not seem to undermine mutual causation there. Newton did use an explicitly stated approximation for calculating the gravity of actual objects by taking the objects as zero-size point objects rather that their actual size. He concluded that experiment proved that using such center-of-gravity points commonly gives an adequate accuracy for gravity calculations, and does not give infinite gravity where two bodies touch as some still falsely claim.
There is often some misunderstanding of Newton's third law of motion - action and reaction or mutual actions being equal and opposite. Does this merely state that the inertia of a body will oppose any force applied to it ? Push reactions can seem simply explained as due to inertia plus exclusive-space-occupancy in a Galileo-Descartes type push-physics. So when A pushes on B with some force-action, B's inertia then pushes back on A with an equal and opposite force-reaction - if bodies actually can contact and push. (What determines the extent to which A and B actually accelerate here, is then the strength of the forces on each relative to the strength of their inertias.) But the equality and oppositeness of attractions or repulsions of bodies separated by some distance may seem to rule out the inertia of B causing any reaction force on A, and somehow require some different action-mutuality so that if A attracts B with some force-attraction then B must attract A with an equal and opposite force-attraction. And for a 'remote-control robot', the 'remote' body can send a signal that causes a physical action in the 'robot' body without there being any physical reaction on the 'remote' body. Newton showed that his laws of motion do apply to gravitating bodies far apart, but was maybe less clear as to exactly how they applied then.
Cartesian physics and all subsequent forms of contact-physics including Einstein's require contact action-reaction to involve zero space separation and imply zero reaction time - ie instantaneous reaction (though zero is not measurable or provable in science experiment). But all Gilbert-Newton attraction physics requires action-reaction to involve positive space separation which implies positive reaction time however small - ie non-instantaneous reaction. Of course opponents of Gilbert-Newton attraction physics have repeatedly falsely claimed the opposite holds, and still do - they nonsensically claim that 'Gilbert-Newton attraction physics requiring instantaneous reaction disproves that physics' !! But some of Newton's physics can seem to assume or require near-instantaneous reactions to forces or near-simultaneous action and reaction - though he nowhere makes even that a specific requirement. But if one (Cartesian) body collides with a second such body then they clearly collide at the same instant and the 2 bodies will seemingly exert collision forces on each other at the same instant, and give equal and opposite changes of momentum to the 2 bodies. Similarly instantaneous reaction also seems required by all push-field or push-continuum theories.
Given an observer body and another body, an observer body clearly generally detects motion in the other body only relative to its own motion and generally detects its own motion only relative to the motion of the other body so that no motion can be determined as being absolute motion. From that it follows that generally neither can absolute motion energies be determined. Laws of relative motions, relative energies and relative forces alone can generally be determined. But if an observer body has some indeterminate motion then other bodies nearby may well share that same motion. And any net motion can be viewed a sum of several different component motions so that any uniform motion might be viewed as composed of (or include) cancelling opposite accelerations, and a motion uniform for a time might yet be begun and/or ended with accelerations. But a net uniform motion can seem to be a net acceleration motion, or viceversa, to an observer body that itself has some appropriate motion. A body can be at rest or in uniform motion when a force is acting on it, only if it is acted on also by some second exactly equal and opposite force. And motion energies, or kinetic energies, are subject to similar requirements.
For an overview of a 'Gilbert-Newton' view of gravity and like forces see The Attraction Theory of gravity and other forces.
The chief evidence of the operation of most physical laws of
nature is found in different motions, as considered in the studies
of many concerning physics such as Galileo, Gilbert, Kepler,
Descartes, Newton and Einstein.
The perseverance of much natural motion like planet orbits helped convince Gilbert and Newton that space offers no resistance to, or drag on, the motion of bodies in it - and cannot affect bodies motion. But both Descartes and Einstein assumed that space can somehow push bodies and so also drag on bodies motion. The perseverance of natural planet orbits seems to some to require at least some steady force such as gravity. However, natural orbits and spins to some seemed like rest and uniform straight line motion in requiring no force to maintain them. And some even thought that uniform straight-line motion does need a force to maintain it.
Spin or rotation of a body about a central fixed point within itself, is commonly considered as for a 'perfectly solid body' or 'uni-part' body though no multi-atom body may actually be such so perhaps little is really known of actual solid body spin. Spin is physically similar to the circular motion of bodies about an external point, as of the Earth and Mars about the Sun, called orbiting or orbital revolution. Both are non-uniform motions that require forces to maintain them as well as to change them - but some forces can be persistent, like the Sun's gravity, and can be internal to a body or a system. If any multi-part object or system held together by limited forces is made to spin fast enough then its parts will fly apart. A 'perfectly solid body' is generally now taken as having parts held together by some infinite force, though short-range strong forces may actually be involved and Descartes-type physics perhaps unreasonably assume some 'uni-part' bodies needing no holding-together forces.
Some natural uniform motion velocities are probably central-attraction escape velocities and probably include atomic escape velocities of which the 'velocity of light' may well be an example. Other major natural uniform motion velocities certainly include those for wave transmission through mediums as for the 'velocity of sound'.
Another basic type of natural motion is deflection or reflection, as where the path of motion of something moving is changed when it meets another object - eg when a moving ball meets a wall or when a light ray meets a mirror. One possible explanation of some or all reflections is contact collision, of two things being unable to occupy the same space so that the parts of any motions directed to occupying the same space have their direction reversed. A second possible explanation of some or all reflections is proximity repulsion, as bodies increasingly repel each other as the distance between them falls. Interestingly for light reflection Newton suggested the further possible explanation of post-contact proximity attraction, where a surface strongly attracts something passing into it and pulls it back out of it. Such case of attraction mimicking repulsion might even also offer an explanation of apparent universe expansion. Of course it is maybe not clear what atomic forces would be needed for that light effect, and Newton might perhaps have done better with a simple repulsion which has attraction mathematics but with an opposite sign. And if billiard ball collisions are in fact proximity repulsions, could the extent of currently known atomic repulsion forces fully explain billiard ball collisions ? And would a perfectly elastic collision require an infinite repulsion force or just repulsion with the inverse square law ? And might post-contact proximity attraction also somehow be able to offer another possible explanation of billiard ball collision ?
It follows from Newton's laws of motion that objects with similar velocities relative to some inertial frame of reference can attain different relative velocities only if forces do different work on them. The 'kinetic energies' of objects are measures of the work required to bring them to rest relative to some inertial frame of reference - and by definition more deceleration being required by a faster object, kinetic energies are the products of objects masses and their velocities relative to the inertial frame of reference. It follows that kinetic energies are not absolute properties of objects, but are only relative properties. But it is generally assumed that objects do have some absolute properties, which might or might not include such things as maybe 'mass' or other properties.
Objects motion can only be changed if some external force is applied to them, and for a given object a greater change in motion requires a greater force being applied. For any two different objects if a given change in motion requires different amounts of force being applied, then they are said to have proportionately different inertias. If the type of force being applied is gravitational force, then they are said to have proportionately different 'masses' or 'gravitational inertias'. But if the type of force being applied is magnetic force, then they are said to have proportionately different 'magnetic powers' or 'magnetic inertias' which will involve both their 'masses' and their 'iron percentage'. But if the type of force applied is 'contact force' or 'momentum force', then the forces and inertias involved are proportionate to the masses and gravitational inertias. Hence an objects inertia relative to gravity and momentum change is commonly called 'its inertia', despite some objects having also different forms of inertia like magnetic inertia. Like all objects non-iron objects have inertias, but they are unaffected by magnetic force with respect to which they hence have infinite inertia. So inertia is basically the responsiveness or non-responsiveness of bodies to forces or force signals. To both Gilbert and Newton, gravity and other like forces are caused by some agent or agents emitted at some high speed by objects and received by or touching other objects.
Motions common in larger visible objects may also be common in less easily seen microscopic objects - or may not. Hence microscopic objects do commonly show one apparently random motion called Brownian motion which may or may not have a real equivalent in larger object motion. And there is always the issue of the absoluteness and the relativity of any motion. Newton saw uniform motion as not distinguishable from a state of rest if the observer had the same uniform motion or state of rest, ie was in the same inertial frame of reference, and from that concluded that an observer could not know if his inertial frame of reference was a state of rest or some undeterminable state of uniform motion. And if gravitation is universal and necessarily non-uniform and accelerating, then maybe nowhere can there really exist any actual inertial frame of reference.
Newton is best know for his work in mathematics, optics and physics, but he certainly owned books on religion, alchemy and other subjects - on which he also wrote much but published little, and many wrongly labeled him an 'alchemist' and a 'heretic Christian', and many still do so, though he did not declare publicly any real belief in either. But, as The Big Bang Theory TV show indicates, many scientists today like Science Fiction and Fantasy Gaming which they know is not real.
In editions of his Opticks from 1706, Newton discussed how microscopic forces analogous to gravity might explain some chemical phenomena and he did publish a little on simple chemistry experiments in the 1730 fourth edition of the Opticks. Newton did many chemistry experiments, but none seemed to have anything to do with the old alchemy aims of making gold or eternal youth. His main science problem almost certainly was to demonstrate exactly how gravity worked and, since magnetism and electricity show different effects with different materials as Gilbert's experiments had shown, he may have sought a substance that would impact gravity differently but found none. Newton maybe was looking for Anti-matter or Dark Matter ?!
Newton knew hot magnets did not attract and, before publishing Principia in 1680 letters to Flamstead, wondered if hot Suns might likewise not attract - clearly seeing gravitational and magnetic or electric forces as having some greater or lesser similarity as Gilbert had. Newton also wondered if emitted electric spirit or signal attraction might be much stronger at atomic distances and be the most likely cause of matter cohering, and such signal attraction physics as best explaining both thinking and unthinking phenomena, in his unpublished manuscript 'The Queries' Questions 24 and 25. And elsewhere Newton wondered if such emissions might be simpler vibration energies, or favoured blackbox no-explanation attraction theory as Qu 23. And Newton did also consider a maybe perverse possible cause of gravity as the emission by bodies of a very rare material medium that got DENSER with distance from its source, so pulling bodies to the less dense area nearer to the source, as in Qy 20, Qu22 and Qu 23. A maybe less perverse explanation of gravity might be a uniform material Ether and bodies emit an Anti-Ether weakening with distance from source that 'eats' such Ether and would do so more nearer bodies and pull bodies to the less dense areas nearer to bodies. Any material Ether would need to be rare enough to give little drag but not so rare as to give little gravity pull, and while the drag would be due to rarities the gravity would be due to rarity gradients and the two not be proportionate so outer planet orbits have more drag. Of course any such simple mechanism that might explain simple gravitational force, is unlikely to be able to also explain trickier magnetic and electric forces (as Einstein's many years of failed unification work showed). It seems that only a physics like Gilbert's signal-response physics can readily handle a variety of differing physical forces and maybe Standard Model 'spins' and 'colour charges'. See www.newtonproject.sussex.ac.uk/
In his Principia, Newton clearly indicated that he considered Rene Descartes to be his main science opponent, and not Robert Hooke who almost all 'Newton historians' have continued to falsely claim was his main science opponent. This and other misrepresenting of Newton they support by ignoring Newton's publications and instead referring to selections of private writings despite Newton being very definite on what of his did or did not merit publishing and public consideration. Unlike some other scientists like Gilbert, Newton's life was not cut short before he ended to his satisfaction his science and its publishing. Further strong unreasonable opposition to Newton lead by catholic Jesuit 'scholars' has been and is still also commonly ignored.
You should be able to read here Sir Isaac Newton's 1687
Philosophiae Naturalis Principia Mathematica (Mathematical
Principles of Natural Philosophy) but somehow the original seems
not available online anywhere. But an online English version is
available and linked to and discussed in our
Newton's Principia section .
Or if you might want to buy Newton books in our USA Newton books or UK Newton books sections.
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