Isaac Newton

He is one of the most important physicists and mathematicians of all time.


This short biography is quoted from the BBC History site.

Isaac Newton was born on 4 January 1643 in Woolsthorpe, Lincolnshire. His father was a prosperous farmer, who died three months before Newton was born. His mother remarried and Newton was left in the care of his grandparents. In 1661, he went to Cambridge University where he became interested in mathematics, optics, physics and astronomy. In October 1665, a plague epidemic forced the university to close and Newton returned to Woolsthorpe. The two years he spent there were an extremely fruitful time during which he began to think about gravity. He also devoted time to optics and mathematics, working out his ideas about ‘fluxions’ (calculus).

In 1667, Newton returned to Cambridge, where he became a fellow of Trinity College. Two years later he was appointed second Lucasian professor of mathematics. It was Newton’s reflecting telescope, made in 1668, that finally brought him to the attention of the scientific community and in 1672 he was made a fellow of the Royal Society. From the mid-1660s, Newton conducted a series of experiments on the composition of light, discovering that white light is composed of the same system of colours that can be seen in a rainbow and establishing the modern study of optics (or the behaviour of light). In 1704, Newton published ‘The Opticks’ which dealt with light and colour. He also studied and published works on history, theology and alchemy.

In 1687, with the support of his friend the astronomer Edmond Halley, Newton published his single greatest work, the ‘Philosophiae Naturalis Principia Mathematica’ (‘Mathematical Principles of Natural Philosophy’). This showed how a universal force, gravity, applied to all objects in all parts of the universe.

In 1689, Newton was elected member of parliament for Cambridge University (1689 – 1690 and 1701 – 1702). In 1696,Newton was appointed warden of the Royal Mint, settling in London. He took his duties at the Mint very seriously and campaigned against corruption and inefficiency within the organisation. In 1703, he was elected president of the Royal Society, an office he held until his death. He was knighted in 1705.

Newton was a difficult man, prone to depression and often involved in bitter arguments with other scientists, but by the early 1700s he was the dominant figure in British and European science. He died on 31 March 1727 and was buried in Westminster Abbey.

Space, Time, and Motion according to Newton

Isaac Newton founded classical mechanics on the view that space is something distinct from body and that time is something that passes uniformly without regard to whatever happens in the world. For this reason he spoke of absolute space and absolute time, so as to distinguish these entities from the various ways by which we measure them (which he called relative spaces and relative times).

Associated with these issues about the ontological status of space and time was the question of the nature of true motion. Newton defined the true motion of a body to be its motion through absolute space.

 It should be emphasized, though, that Newton did not regard space and time as genuine substances (like bodies and minds), but rather as real entities which are created in their own manner according to God’s divine will.

In his book Philosophiæ Naturalis Principia Mathematica from 1687 he lays out his views on time, space, place, and motion. He begins by saying that, since in common life these quantities are conceived in terms of relations to sensible bodies, it is incumbent to distinguish between, on the one hand, the relative, apparent, common conception of them, and, on the other, the absolute, true, mathematical quantities themselves. To paraphrase:

  • Absolute, true, and mathematical time, from its own nature, passes equably without relation the anything external, and thus without reference to any change or way of measuring of time (e.g., the hour, day, month, or year).
  • Absolute, true, and mathematical space remains similar and immovable without relation to anything external. (This contrasts to to Descartes’ concept of space.) Relative spaces are measures of absolute space defined with reference to some system of bodies or another, and thus a relative space may, and likely will, be in motion.
  • The place of a body is the space which it occupies, and may be absolute or relative according to whether the space is absolute or relative.
  • Absolute motion is the translation of a body from one absolute place to another; relative motion the translation from one relative place to another.

Newton argues that the distinction between the true quantities and their relative measures is necessary and justified.

Therefore, according to Newton:

  1. space is something distinct from body and exists independently of the existence of bodies,
  1. It is a true fact when a given body moves and what its true quantity of motion is, and
  1. the true motion of a body does not consist of, or cannot be defined in terms of, its motion relative to other bodies.

The first point was massively contested in 17th-century natural philosophy.  Leibniz, Huygens, and Berkeley attacked him for it. The second was not in general dispute.

Newton’s Laws of Motion

Newton’s laws of motion are three physical laws that form the basis for classical mechanics. They describe the relationship between the forces acting on a body and the motion that results from these forces. They have been expressed in several ways over nearly three centuries, and can be summarized as follows:

  1. First law: The velocity of a body remains constant unless the body is acted upon by an external force.
  2. Second law: The acceleration (a) of a body is parallel and directly proportional to the net force (F) and inversely proportional to the mass (m), therefore: F = ma.
  3. Third law: The mutual forces of action and reaction between two bodies are equal, opposite and collinear.

The three laws of motion were published in 1687. Newton used them to explain and investigate the motion of many physical objects and systems. For example,  Newton showed that these laws of motion, combined with his law of universal gravitation, explained Kepler’s laws of planetary motion.

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