Our present idea about the motion of bodies date back to Galileo and
Newton. Before them people believed Aristotle, who said that the
natural state of a body was to be at rest and that it moved only if
driven by a force or impulse. It followed that a heavy body should
fall faster than a light one, because it would have a greater pull
toward the earth.
The Aristotelian tradition also held that one could work out all the
laws that govern the universe by pure thought; it was not necessary to
check by observation. So no one until Galelio bothered to see whether
bodies of different weight did in fact fall at different speeds. It is
said that Galelio demonstrated that Aristotle's belief was false by
dropping weights from the leaning tower of Pisa. The story is almost
certainly untrue, but Galelio did do something equivalent; he rolled
balls of different weights down a smooth slope. The situation is
similar to the that of heavy bodies falling vertically, but it is easier
to observe because the speeds are smaller. Galelio's measurements
indicated that each body increased its speed at the same rate, no
matter what its weight. Galileo's measurements were used by Newton as
the basis of his laws of motion. In Galileo's experiments,as a body
rolled down the slope it was always acted on by the same force. (its
weight), and the effect was to make it constantly speed up. This shows
that the real effect of a force is to change the speed of a body,
rather than just to set it moving , as was previously thought. It also
meant that whenever a body was not acted upon by any force it will
keep on moving in a straight line at the same speed. This idea was
first stated explicitly in Newton's Principia Mathematica first
published in 1687, and is known as Newton's first law. What happens to
a body when a force acts on it is given by Newton's second law. This
states that a body will accelerate or change its speed, at a rate that
is proportional to the force. The acceleration is also smaller the
greater the mass of the body.
In addition to the laws of motion, Newton discovered a law to describe
the force of gravity, which states that every body attracts every other
boddy with a force that is proportional to the mass of each body. Thus
the force between two bodies would be twice as strong if one of the
bodies (say, body A) had its mass doubled. This is what you might expect
because one could think of the new body A as being made of two bodies
with the original mass. Each would attract body B with the original
force. Thus the total force between A and B would be twice the original
force. And if, say, one of the bodies had twice the mass, and the other
had three times the mass, then the force would be six times as strong.
One can now see why all bodies fall in the same rate; a body of twice
the weight will have twice the force of gravity pulling it down, but
it will also have twice the mass. According to Newton's second law,
these two effects will exactly cancel each other, so the acceleration
will be the same in all cases.
