# Newton, Sir Isaac

### newton’s force body motion

(1642–1727) English physicist and mathematician: discovered the binomial theorem, invented calculus and produced theories of mechanics, optics and gravitation.

Isaac Newton: his earliest portrait, at age 46, by Sir Godfrey Kneller. Newton was writing the *Principia* at about this time. Many later portraits exist, although Newton claimed to detest being painted.

Newton was born prematurely in the year died, 3 months after the death of his father, the owner of Woolsthorpe Manor in Lincolnshire. He was left in the care of his grandmother at Woolsthorpe when his mother remarried, and came under the influence of his uncle, who recognized his talents. Newton went to the grammar school in Grantham and after farming at Woolsthorpe for 2 years was sent to Trinity College, Cambridge, in 1661. He remained there for nearly 40 years. It is clear that as a young man Newton was what we would now describe as rather a hippy. A contemporary recalled that he often ‘dined in college … stockings untied, head scarcely combed’. His portrait by Kneller, made when he was 46, still shows him in casual dress and with very long hair.

As a student Newton attended Isaac Barrow’s (1630–77) lectures on mathematics. In 1665 the Great Plague caused him to return to his isolated home at Woolsthorpe. Here he worked on many of the ideas for which he is so famous, during what became known as his ‘miraculous year’. Later ( *c* .1716) Newton wrote in his notebooks:

‘In the beginning of the year 1665 I found the method for approximating series and the rule for reducing any dignity [power] of any binomial to such a series [ie the binomial theorem]. The same year in May I found the method of tangents of Gregory and Sulzius, and in November had the direct method of Fluxions [ie the elements of the differential calculus], and in the next year in January had the Theory of Colours, and in May following I had entrance into the inverse method of Fluxions [ie integral calculus], and in the same year I began to think of gravity extending to the orb of the Moon … and … compared the force requisite to keep the Moon in her orb with the force of gravity at the surface of the Earth…. All this was in the two years of 1665 and 1666, for in those years I was in the prime of my age for invention, and minded Mathematics and Philosophy more than at any time since.’

On returning to Trinity College, he was elected a Fellow (1667) and succeeded Isaac Barrow as Lucasian Professor in 1669 at the age of 26. He was made a Fellow of the Royal Society in 1672. During 1669–76 Newton presented many of his results in optics and became engaged in controversies concerning them. In 1679 he began to correspond with , renewing his interest in dynamics and solving the problem of elliptical planetary motion discovered by visited Newton in 1684 and persuaded him to write a work on dynamics, which was written within 18 months; his *Philosophiae naturalis principia mathematica* (1687, The Mathematical Principles of Natural Philosophy) – the ‘ *Principia* ’. It is the most important and influential scientific book ever written.

From that point Newton’s mathematical interests waned, giving place to theology (ironically, bearing in mind his college, he seems to have been anti-trinitarian) and involvement in political life. He also spent much time and effort on alchemy, without result. In 1687 he courageously accompanied the vice-chancellor to London to defend the university against illegal encroachments by James II. In 1692 Newton ‘lost his reason’, as he phrased it; probably he suffered a period of severe depression.

Then his interests turned to London and, via his friendship with Charles Montague, Fellow of the Royal Society and first earl of Halifax, Newton became Warden and then Master of the Mint in 1696 and 1698 respectively, skilfully reforming the currency. He was knighted for this in 1705. In London, a young niece became his housekeeper. She was Catherine Barton, described as beautiful, charming and witty, and the mistress of Charles Montague. Newton’s duties at the Mint included supervising the recoinage and, rather oddly, the capture, interrogation and prosecution of counterfeiters.

In 1701 he resigned the Lucasian professorship and his fellowship at Trinity, although he remained president of the Royal Society from 1703 until his death. He was elected a Whig member of Parliament for the university, but was not very active politically.

Much of Newton’s last 20 years were spent in acrimonious debate over priority in scientific discoveries with , and in this Newton showed both ruthlessness and obsessiveness. Following a painful illness (due to a gallstone), he died in 1727, and is buried in Westminster Abbey. He had been remarkably fit, lacking only one tooth and not needing spectacles, even in old age, and reasonably wealthy. He seems to have had no romantic or imaginative life outside science. Asked his opinion on poetry, he replied ‘I’ll tell you that of Barrow; he said that poetry was a kind of ingenious nonsense.’

Newton’s researches on mechanics display great mastery and established a uniform system based on the three laws of motion: (1) a body at rest or in uniform motion will continue in that state unless a force is applied; (2) the applied force equals the rate of change of momentum of the body; (3) if a body exerts a force on another body there is an equal but opposite force on the first body. From these Newton explained the collision of particles, Galileo’s results on falling bodies, Kepler’s three laws of planetary motion and the motion of the Moon, Earth and tides. The deductions were made using calculus, but were proved geometrically in the *Principia* to clarify it for contemporary readers. The general theory of gravitation, that any two bodies of mass *m* 1 , *m* 2 at a distance *d* apart, attract each other with a force *F* :

where *G* is a universal constant, was developed by Newton from his original work on the Moon’s motion of 1665. It may well be correct (as Newton’s niece maintained) that the idea stemmed from seeing an apple fall from a tree beside his Woolsthorpe home. Three centuries ahead of the technology he showed that the escape speed *s* of an artificial satellite from a planet of mass *m* and radius *r* is given by *s* =(2 *Gm* / *r* ) ½ . At speeds less than this, the projected satellite will return to the planetary surface.

Newton published another celebrated treatise, the *Opticks* of 1704, which was an organized and coherent account of the behaviour of light. Based on his own ingenious experimental work, it proposed a corpuscular theory of light but added ideas of periodicity (which were missing even in Hooke’s and wave theory). Such phenomena as refraction of light by a prism (with the production of colours by dispersion) and Newton’s rings of coloured light, about the point of contact between a lens and mirror, were considered. Also named after him is the Newtonian telescope, which used mirrors rather than lenses to gather light and achieve magnification

Newton’s telescope, the first reflector. Made entirely by him and about 6 inches long; it was presented to the Royal Society in 1671. The two crowns are a single decoration on a building over 300 ft distant, as seen (Fig 2) through Newton’s reflector (giving 38×), and (in Fig 3) with colour fringes through a 25-inch long refracting telescope.

Newton’s name is linked with a variety of matters in physics, in addition to those already noted (eg the laws of motion). Thus the SI unit of force, the newton (N), is based on the second law, in the form which defines the force *F* which produces a constant acceleration *a* in a body of mass *m* , by the relation *F* = *ma* . The newton is the force which produces an acceleration of 1 m s –2 when it acts on a mass of 1 kg.

In fluid mechanics, Newtonian fluids are those whose viscosity is independent of the rate of shear or the velocity gradient. Colloids and some other solutions form non-Newtonian fluids. Newton’s law of cooling states that the rate at which a body loses heat to its surroundings is proportional to the temperature difference between the body and its surroundings. It is empirical, and applies only to small differences of temperature, and to forced convection.

Newton exerted a unique and profound influence on science and thought. As a mathematician he discovered the binomial theorem and the calculus; the latter, together with his law of universal gravitation are the peaks of his achievement and the basis of his colossal stature. His work established the scientific method and placed physics on a new course, giving mathematical expression to physical phenomena and permanently altering modern thought.

Einstein wrote of him: ‘Nature was to him an open book, whose letters he could read without effort…. In one person he combined the experimenter, the theorist, the mechanic and, not least the artist in exposition. He stands before us strong, certain and alone: his joy in creation and his minute precision are evident in every word and every figure.’

Newton’s view of himself at the end of his life has a different emphasis: ‘I do not know what I may appear to the world; but to myself I seem to have been only like a boy playing on the seashore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.’ Some have suggested this was conventional false modesty.

His birthplace, Woolsthorpe Manor, is maintained by the National Trust.

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