Sir Isaac Newton (25 December 1642 [NS: 4 January 1643] – 20 March 1727 [NS: 31 March 1727]) was an English physicist, mathematician, astronomer, natural philosopher, alchemist and theologian, who has been considered by many to be the greatest and most influential scientist who ever lived. Although his career was long and littered with success, there were four discoveries that were considered to be his most important.
Law of universal gravitation
Thoughts of gravitation entered Newton’s head as result of a certain apple tree and the tree’s falling fruit. In 1666, while Newton was sitting in the manor house garden at Woolsthorpe, he saw an apple fall from a tree. This triggered certain thoughts that he had been having about gravitation. Despite popular belief, the apple did not fall on his head. What actually happened was that he saw an apple fall from an apple tree and he began to wonder why it fell. From there his thoughts broadened to the rotation of the moon. It was already common knowledge that the moon revolved around the Earth and the planets revolved around the sun. This was caused by gravity. What Newton wanted to know was why the moon revolved around the earth instead of simply being pulled into the earth like the apple was. This brainstorm (which some scholars suspect Newton may have invented late in life) ultimately led to his law of universal gravitation. The law says that all particles of matter in the universe attract every other particle, that gravitational attraction is a property of all matter. The law explained many things, from the orbits of the planets around the sun to the influence of the moon and sun on the tides. And it held sway as the accepted description of terrestrial and celestial mechanics for almost 200 years, until Einstein came along and rocked the boat with relativity.
Three 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 its motion due to those forces. They have been expressed in several different ways over nearly three centuries and can be summarized as follows:
- First law: If an object experiences no net force, then its velocity is constant: the object is either at rest (if its velocity is zero), or it moves in a straight line with constant speed (if its velocity is nonzero).
- Second law: The acceleration a of a body is parallel and directly proportional to the net force F acting on the body, is in the direction of the net force, and is inversely proportional to the mass m of the body, i.e., F = ma.
- Third law: When a first body exerts a force F1 on a second body, the second body simultaneously exerts a force F2 = −F1 on the first body. This means that F1 and F2 are equal in magnitude and opposite in direction.
Theory of light and color
Newton became stuck while trying to figure out what the radius of the earth was in order to help him prove his Universal Law of Gravitation. Rather than guess and take a chance that he might be wrong, he decided to put the project on hold and study something else. That something else optics, or the study of color and light. From 1670 to 1672, Newton lectured on optics. During this period he investigated the refraction of light, demonstrating that a prism could decompose white light into a spectrum of colours, and that a lens and a second prism could recompose the multicoloured spectrum into white light.
He also showed that the coloured light does not change its properties by separating out a coloured beam and shining it on various objects. Newton noted that regardless of whether it was reflected or scattered or transmitted, it stayed the same colour. Thus, he observed that colour is the result of objects interacting with already-coloured light rather than objects generating the colour themselves.
From this work, he concluded that the lens of any refracting telescope would suffer from the dispersion of light into colours (chromatic aberration). As a proof of the concept, he constructed a telescope using a mirror as the objective to bypass that problem.
When Newton began to muse on the problem of the motion of the planets and what kept them in their orbits around the sun, he realized that the mathematics of the day weren’t sufficient to the task. Properties such as direction and speed, by their very nature, were in a continuous state of flux, constantly changing with time and exhibiting varying rates of change. So he invented a new branch of mathematics, which he called the fluxions (later known as calculus). Calculus allowed him to draw tangents to curves, determine the lengths of curves, and solve other problems that classical geometry could not help him solve. Interestingly, Newton’s masterwork, the Principia, doesn’t include the calculus in the form that he’d invented years before, simply because he hadn’t yet published anything about it. But he did combine related methods with a very high level of classical geometry, making no attempt to simplify it for his readers. The reason was, he said, “to avoid being baited by little Smatterers in Mathematicks.”