The existence of life itself has been attributed over the ages to an underlying "force". Life is manifested by change and movement - it involves actions and interactions of a variety of forces. Therefore, no measurement is more fundamental to human activity than the measurement of force in its many manifestations, including weight, pressure, acceleration, torque, work, and energy.
The purpose of this first chapter is to trace the historical evolution of the understanding of force and of the theories which evolved at various stages of human development. While the ancient civilizations of 8,000 to 6,000 B.C., in the river valleys of Southwest Asia, Mesopotamia or Egypt and others in China, India, and South America, all used lever and roller systems to amplify the muscle power of men, the first attempts to formalize a theoretical understanding of force were in ancient Greece.
From Aristotle to Hawking
The ancient Greek philosophers considered themselves qualified to make pronouncements in the field of science, but their views had little to do with the real world. Aristotle (384-322 B.C.), for example, believed that "form" caused matter to move. He defined motion as the process by which the "potentiality" of matter became the "actuality" of form. With that view of reality, it is no wonder that the Greeks of Aristotle's time created much more art than technology.
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Yet, a hundred years later, the Greek physicist Archimedes (287-212 B.C.) became a pioneer of real engineering experimentation. He not only discovered the force-amplifying capability of the pulley, but also noted that the same weight of gold will displace less water than does an equal weight of silver.
Some 400 years later, the astronomer Clausius Ptolemaeus (second century A.D.) developed the first model of planetary movements. He assumed the Earth as being stationary in the center of the universe, with the Sun, Moon and stars revolving around it in circular orbits. The first revision of the Ptolemaic system came a millennium later; Nicholas Copernicus (1473-1543) replaced the Earth with the Sun as the center of the universe (a heliocentric system). Because he still did not understand the role of the force of gravity, however, he, too, assumed that the planets traveled in perfect circles.
Another century passed before Galileo Galilei (1564-1642) discovered, by dropping various items from the. Leaning Tower of Pisa, that the velocity of a falling object is independent of its weight. His attitude was that of a good engineer: "I don't know why, but it works, so don't forget it!"
Johannes Kepler (1571-1630), who correctly established that the orbits of the planets about the Sun are elliptical, did not realize the cause of all this: the force of gravity. He noted that the Sun had some 'mysterious power or virtue' which compelled the planets to hold to their orbits. The role of gravity escaped even Blaise Pascal (1623-1662), although he did correctly explain some related phenomena such as pressure and barometric pressure. It was also Pascal who first noted that, when pressure is applied to a confined fluid, the pressure is transmitted undiminished in all directions. It is for these discoveries that we honor him by using his name (in the SI system) as the unit of pressure.
The role of the force of gravity was first fully understood by Sir Isaac Newton (1642-1727). His law of universal gravitation explained both the fall of bodies on Earth and the motion of heavenly bodies. He proved that gravitational attraction exists between any two material objects. He also noted that this force is directly proportional to the product of the masses of the objects and inversely proportional to the square of the distance between them. On the Earth's surface, the measure of the force of gravity on a given body is its weight. The strength of the Earth's gravitational field (g) varies from 9.832 m/sec2 at sea level at the poles to 9.78 m/sec2 at sea level at the Equator.
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