Great Physicists

MECHANICS
Historical Synopsis


Physics builds from observations. No physical theory can succeed if
it is not confirmed by observations, and a theory strongly supported
by observations cannot be denied. For us, these are almost truisms.
But early in the seventeenth century these lessons had not yet been
learned. The man who first taught that observations are essential and
supreme in science was Galileo Galilei.
Galileo first studied the motion of terrestrial objects, pendulums,
free-falling balls, and projectiles. He summarized what he observed
in the mathematical language of proportions. And he extrapolated
from his experimental data to a great idealization now called the
“inertia principle,” which tells us, among other things, that an object
projected along an infinite, frictionless plane will continue forever at
a constant velocity. His observations were the beginnings of the
science of motion we now call “mechanics.”
Galileo also observed the day and night sky with the newly
invented telescope. He saw the phases of Venus, mountains on the
Moon, sunspots, and the moons of Jupiter. These celestial
observations dictated a celestial mechanics that placed the Sun at
the center of the universe. Church doctrine had it otherwise: Earth
was at the center. The conflict between Galileo’s telescope and
Church dogma brought disaster to Galileo, but in the end the
telescope prevailed, and the dramatic story of the confrontation
taught Galileo’s most important lesson.
Galileo died in 1642. In that same year, his greatest successor,
Isaac Newton, was born. Newton built from Galileo’s foundations a
system of mechanics based on the concepts of mass, momentum,
and force, and on three laws of motion. Newton also invented a
mathematical language (the “fluxion” method, closely related to our
present-day calculus) to express his mechanics, but in an odd
historical twist, rarely applied that language himself.
Newton’s mechanics had—and still has—cosmic importance. It
applies to the motion of terrestrial objects, and beyond that to
planets, stars, and galaxies. The grand unifying concept is Newton’s
theory of universal gravitation, based on the concept that all objects,

small, large, and astronomical (with some exotic exceptions), attract
one another with a force that follows a simple inverse-square law.
Galileo and Newton were the founders of modern physics. They
gave us the rules of the game and the durable conviction that the
physical world is comprehensible.