the motion ceases. The true Newtonian law takes diametrically the opposite point of view. The state of a body unacted on by force is that of uniform motion in a straight line, and no external force or influence is to be looked for as the cause, or, if you like to put it so, as the invariable accompaniment of this uniform rectilinear motion. Rest is merely a particular case of such motion, merely when the velocity is and remains zero. Thus, when a body is moving, we do not seek for any external influence except to explain changes in the rate of the velocity or changes in its direction. So long as the body is moving at the
same rate and in the same direction there is no need to invoke the aid of any forces.
The difference between the two points of view is well seen by reference to the theory of the motion of the planets. Copernicus, a
Pole, born at Thorn in West Prussia (born 1473, died 1543), showed how much simpler it was to conceive the planets, including the 4 earth as revolving round the sun in orbits which are nearly circular; and later, Kepler,
a German mathematician, in the year 1609 proved that, in fact, the orbits are practically ellipses, that is, a special sort of oval curves
which we will consider later in more detail. Immediately the question arose as to what are the forces which preserve the planets in this motion. According to the old false view,
held by Kepler, the actual velocity itself required
preservation by force. Thus he looked for tangential forces as in the accompanying figure ([fig:4]4). But according to the Newtonian law, apart from some force the planet would move for ever with its existing velocity in a straight line, and thus depart entirely from the sun. Newton, therefore, had to search
for a