1.4.1 Gravitational Force
The gravitational force is the force of mutual
attraction between any two objects by virtue of
their masses. It is a universal force. Every object
experiences this force due to every other object
in the universe. All objects on the earth, for
example, experience the force of gravity due to
the earth. In particular, gravity governs the
motion of the moon and artificial satellites around
the earth, motion of the earth and planets
around the sun, and, of course, the motion of
bodies falling to the earth. It plays a key role in
the large-scale phenomena of the universe, such
as formation and evolution of stars, galaxies and
galactic clusters.
1.4.2 Electromagnetic Force
Electromagnetic force is the force between
charged particles. In the simpler case when
charges are at rest, the force is given by
Coulomb’s law : attractive for unlike charges and
repulsive for like charges. Charges in motion
produce magnetic effects and a magnetic field
gives rise to a force on a moving charge. Electric
and magnetic effects are, in general,
inseparable – hence the name electromagnetic
force. Like the gravitational force,
electromagnetic force acts over large distances
and does not need any intervening medium. It
is enormously strong compared to gravity. Theelectric force between two protons, for example,
is 1036 times the gravitational force between
them, for any fixed distance.
Matter, as we know, consists of elementary
charged constituents like electrons and
protons. Since the electromagnetic force is so
much stronger than the gravitational force, it
dominates all phenomena at atomic and
molecular scales. (The other two forces, as we
shall see, operate only at nuclear scales.) Thus
it is mainly the electromagnetic force that
governs the structure of atoms and molecules,
the dynamics of chemical reactions and the
mechanical, thermal and other properties of
materials. It underlies the macroscopic forces
like ‘tension’, ‘friction’, ‘normal force’, ‘spring
force’, etc.
Gravity is always attractive, while
electromagnetic force can be attractive or
repulsive. Another way of putting it is that mass
comes only in one variety (there is no negative
mass), but charge comes in two varieties :
positive and negative charge. This is what
makes all the difference. Matter is mostly
electrically neutral (net charge is zero). Thus,
electric force is largely zero and gravitational
force dominates terrestrial phenomena. Electric
force manifests itself in atmosphere where the
atoms are ionised and that leads to lightning.If we reflect a little, the enormous strength
of the electromagnetic force compared to
gravity is evident in our daily life. When we
hold a book in our hand, we are balancing the
gravitational force on the book due to the huge
mass of the earth by the ‘normal force’
provided by our hand. The latter is nothing
but the net electromagnetic force between the
charged constituents of our hand and
the book, at the surface in contact. If
electromagnetic force were not intrinsically so
much stronger than gravity, the hand of the
strongest man would crumble under the
weight of a feather ! Indeed, to be consistent,
in that circumstance, we ourselves would
crumble under our own weight !
1.4.3 Strong Nuclear Force
The strong nuclear force binds protons and
neutrons in a nucleus. It is evident that without
some attractive force, a nucleus will be
unstable due to the electric repulsion between
its protons. This attractive force cannot be
gravitational since force of gravity is negligible
compared to the electric force. A new basic force
must, therefore, be invoked. The strong nuclear
force is the strongest of all fundamental forces,
about 100 times the electromagnetic force instrength. It is charge-independent and acts
equally between a proton and a proton, a
neutron and a neutron, and a proton and a
neutron. Its range is, however, extremely small,
of about nuclear dimensions (10–15m). It is
responsible for the stability of nuclei. The
electron, it must be noted, does not experience
this force.
Recent developments have, however,
indicated that protons and neutrons are built
out of still more elementary constituents called
quarks.
