Science 101: Physics

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Chapter One

Newton, Motion, and Classical Mechanics

The early Greek philosophers made several important advances in our understanding of the natural world, but most of what they did was abstract philosophizing. They did not believe in experimentation and felt that all answers could be found through pure reason. It was not until the sixteenth century that someone decided to test the ideas and assumptions that had to that point been accepted as fact. Galileo Galilei (1564-1642) realized that all beliefs about our world had to be proven before they could be accepted. He did not accept, for example, the idea that heavy objects fell faster than light ones, and experimentation soon showed him he was right. Galileo started a revolution in physical science, but it was Sir Isaac Newton (1642-1727) of England who brought it to fruition and put it on a firm foundation.

Newton's intellect changed science forever and gave us what we call classical mechanics, the branch of physics that studies the deterministic motion of objects.

Early Ideas

Many early scholars no doubt wondered about the physical phenomenon we know as motion. Fortunately, some of the records from these Greek philosophers have survived, so we know their thoughts on many types of motion: the motion of stones that were dropped or thrown, the motion of objects that were pushed or pulled, the motion of flames and smoke, and the motion of bubbles in water, to name a few. One of the most influential of these early philosophers was Aristotle, who lived from 384 to 322 BCE.

Aristotle

Aristotle believed that the Earth wasmade up of four elements—earth, water, air, and fire—and that each of these elements had its "natural" place according to its weight. The element earth was heaviest, so it was closest to the center of the planet. Directly above earth was water, then air, and finally at the outer edge was fire.

Within this scheme, motion was also a natural phenomenon. If you raised a rock and dropped it, it fell through the air to its natural place, namely the ground (or Earth). It was understood, however, that a rock could be thrown upward, which did not appear to be a natural motion. Aristotle explained this by saying that the impulse given to the stone was transmitted to the air, and the air carried the stone. But it only carried it a small distance before natural motion took over and brought it back to Earth.

Some things had to rise to achieve their natural place. Bubbles rose through water, and flames rose through air. Aristotle also had an explanation for motion in the heavens. Heavenly objects such as the Sun, Moon, and stars were not composed of the same elements as those on Earth, so they did not behave in the same manner. These objects were composed of a fifth element called the fifth essence, or aether. The natural motion of heavenly objects was in circles around the center of the universe, and the Earth was the center of the universe. Some of the heavenly bodies, namely stars, appeared to be at rest. Aristotle had no explanation for these celestial objects.

Galileo

No one challenged Aristotle's ideas of motion and the makeup of the Earth for almost 2,000 years. The first individual on record to test these concepts was the Italian physicist and mathematician Galileo Galilei. Born in Pisa, Italy, in 1564, Galileo was skeptical of many of Aristotle's teachings and eventually decided to prove them for himself.

Aristotle had put forward the idea that heavier objects fell at a faster rate than lighter objects. He pointed to leaves and snowflakes, comparing them to stones. Yet he never tested his assumptions. Galileo began by considering a cannonball. According to Aristotle, if you dropped the ball it would fall at one rate, but if you cut it in half the two halves would fall at much slower rates. Galileo was sure this was wrong. After a few simple experiments, he realized that air was playing a significant role and came to the conclusion that if the air was absent, everything would fall at the same rate. Unfortunately, he had no way of proving this, because he could not create a vacuum.

Sir Isaac Newton

Born prematurely in Woolsthorpe, England, on Christmas Day, 1642, Isaac Newton almost died at birth. His father had previously passed away, and when his mother remarried three years later he was sent to live with his grandparents. Young Isaac was not an outstanding student, preferring to spend his time building windmills and water clocks. His mother eventually decided to take him out of school so he could help on the family farm, but it was soon obvious he was not suited for this type of work. He was usually found reading under a tree when he was supposed to be working. Several of Isaac's teachers eventually persuaded his mother to send him to Trinity College at Cambridge.

Newton entered the college in 1660 and graduated in 1664. Little is known about his academic record, but he did attract the attention of one of his teachers, Isaac Barrow. In 1665 the plague arrived in England and Newton was forced to return to his mother's farm at Woolsthorpe. During this time he made several important discoveries, including the laws of motion and the law of gravity. It was also at Woolsthorpe that Newton developed calculus.

Calculus

One of Newton's most important contributions was the development of the powerful mathematical tool that he referred to as the "method of fluxions." It was important in dealing with instantaneous rates of change, and also in calculating areas and volumes. The same ideas in a slightly different form were discovered about the same time by Gottfried Leibniz in Germany. A controversy eventually arose when friends of Newton in England began claiming that Leibniz had stolen Newton's idea. There is little doubt that Newton came up with the idea first, but he was slower . . .