GRAVITY
BY:ANDY HELMSTETTER
Newton's more specific contribution to the description of the
forces in nature was the elucidation of the force of gravity. Today scientists
know that in addition to gravity only three other fundamental forces give rise
to all observed properties and activities in the universe: those of
electromagnetism, the so-called strong nuclear interactions that bind together
the neutrons and protons within atomic nuclei, and the weak interactions between
some of the elementary particles that account for the phenomenon of
radioactivity. Understanding of the force concept, however, dates from the
universal law of gravitation, which recognizes that all material particles, and
the bodies that are composed of them, have a property called gravitational mass.
This property causes any two particles to exert attractive forces on each other
(along the line joining them) that are directly proportional to the product of
the masses, and inversely proportional to the square of the distance between the
particles. This force of gravity governs the motion of the planets about the sun
and the earth's own gravitational field, and it may also be responsible for the
possible gravitational collapse, the final stage in the life cycle of stars.
One of the most important observations of physics is that the
gravitational mass of a body (which is the source of one of the forces existing
between it and another particle), is effectively the same as its inertial mass,
the property that determines the motional response to any force exerted on it .
This equivalence, now confirmed experimentally to within one part in 1013, holds
in the sense of proportionality—that is, when one body has twice the
gravitational mass of another, it also has twice the inertial mass. Thus,
Galileo's demonstrations, which antedate Newton's laws, that bodies fall to the
ground with the same acceleration and hence with the same motion, can be
explained by the fact that the gravitational mass of a body, which determines
the forces exerted on it, and the inertial mass, which determines the response
to that force, cancel out.
The full significance of this equivalence
between gravitational and inertial masses, however, was not appreciated until
Albert Einstein, the theoretical physicist who enunciated the theory of
relativity, saw that it led to a further implication: the inability to
distinguish between a gravitational field and an accelerated frame of reference.
The force of gravity is the weakest of the four forces of nature when
elementary particles are considered. The gravitational force between two
protons, for example, which are among the heaviest elementary particles, is at
any given distance only 10-36 the magnitude of the electrostatic forces between
them, and for two such protons in the nucleus of an atom, this force in turn is
many times smaller than the strong nuclear interaction. The dominance of gravity
on a macroscopic scale is due to two reasons: (1) Only one type of mass is
known, which leads to only one kind of gravitational force, which is attractive.
The many elementary particles that make up a large body, such as the earth,
therefore exhibit an additive effect of their gravitational forces in line with
the addition of their masses, which thus become very large. (2) The
gravitational forces act over a large range, and decrease only as the square of
the distance between two bodies.
By contrast, the electric charges of
elementary particles, which give rise to electrostatic and magnetic forces, are
either positive or negative, or absent altogether. Only particles with opposite
charges attract one another, and large composite bodies therefore tend to be
electrically neutral and inactive. On the other hand, the nuclear forces, both
strong and weak, are extremely short range and become hardly noticeable at
distances of the order of 1 million-millionth of an inch.
Despite its
macroscopic importance, the force of gravity remains so weak that a body must be
very massive before its influence is noticed by another. Thus, the law of
universal gravitation was deduced from observations of the motions of the
planets long before it could be checked experimentally. Not until 1771 did the
British physicist and chemist Henry Cavendish confirm it by using large spheres
of lead to attract small masses attached to a torsion pendulum, and from these
measurements also deduced the density of the earth.
In the two centuries
after Newton, although mechanics was analyzed, reformulated, and applied to
complex systems, no new physical ideas were added. The Swiss mathematician
Leonhard Euler first formulated the equations of motion for rigid bodies, while
Newton had dealt only with masses concentrated at a point, which thus acted like
particles. Various mathematical physicists, among them Joseph Louis Lagrange of
France and Sir William Rowan Hamilton of Ireland extended Newton's second law in
more sophisticated and elegant reformulations. Over the same period, Euler, the
Dutch-born scientist Daniel Bernoulli, and other scientists also extended
Newtonian mechanics to lay the foundation of fluid mechanics.
