Mechanics, Planetary Motion and the Modern Science Revolution 57
weaker than the Earth’s. The mass of the object on the Earth and on the
Moon is the same, however, even though their weights are different. The
velocity of a particle describes both its speed i.e. the distance covered per
unit time and its direction of motion. A body with a constant velocity is
therefore one moving with a uniform rate of speed along a straight line.
A planet orbiting a star in a perfect circular motion at constant speed
does not have a constant velocity because it is continually changing
its direction. Anybody whose velocity is undergoing change i.e. not
remaining constant is said to be undergoing acceleration. The term
acceleration in non-technical parlance usually means to speed up. For
the physicist however the word has a more general meaning so that a
body that is either speeding up, slowing down or changing direction is
undergoing acceleration.
Although the maintenance of constant velocity motion requires
no explanation in the Newtonian system all accelerations must be
explained in terms of some force. The motion of a body does not change
spontaneously; some other second body must be exerting a force to
produce the change in motion of the first body. For example Newton
explained the change in motion of the planets as they orbited the Sun as
due to the gravitational attraction between the Sun and the planets or the
acceleration of the proverbial apple that hit him upon the head as due to
the gravitational attraction between the Earth and the apple. In these two
examples the gravitational force is responsible for the change of motion.
The force that one body exerts on another in order to change its motion,
however, can also arise as a result of their colliding and transferring
their momentum as occurs when two billiard balls collide and change
direction.
Newton discovered in two body interactions, independent of the
nature of the force, that the force that the first body exerts on the second
body is equal and opposite to the force the second body exerts on the
first. This concept is illustrated by considering a person diving from a
raft. By pushing against the raft he dives forward into the water. The raft
does not stand still but floats backwards. The force the raft exerts on the
person propels him forward. The equal and opposite force of the person
on the raft makes the raft float backward.
The same effect is illustrated by the fact that when a gun is discharged
it recoils in the direction opposite to which the bullet is fired. The force
of the bullet on the gun is equal and opposite to the force of the gun on
the bullet. The reason that the speed of the bullet is so much greater than