A View of Sir Isaac Newton's PhilosophyPemberton, Henry
Science
A View of Sir Isaac Newton's Philosophy
Pemberton, Henry
Newton, Isaac, 1642-1727. Principia
14. IN like manner we may determine the nature of the motion, if
the line, wherein the body sets out, be inclined (as in fig. 76.)
down toward the line B A drawn between the body and the center. If
the centripetal power so much increases in strength, as the body
approaches, that it can bend the path, in which the body moves, to
that degree, as to cause all the lines as A H, A I, A K to remain no
less oblique to the motion of the body, than A B is oblique to B C;
the body shall continually more and more approach the center. But if
the centripetal power increases in so much less a degree, as to permit
the line drawn from the center to the body, as it accompanies the
body in its motion, at length to become more and more erect to the
curve wherein the body moves, and in the end, suppose at K, to become
perpendicular thereto; from that time the body shall rise again. This
is evident from what has been said above; because for the very same
reason here also the body shall proceed from the point K to describe a
line altogether similar to the line, in which it has moved from B to K.
Thus, as it was observed of the pendulum in the preceding chapter[88],
that all the time it approaches towards being perpendicular to the
horizon, it more and more descends; but, as soon as it is come into
that perpendicular situation, it immediately rises again by the same
degrees, as it descended by before: so here the body more and more
approaches the center all the time it is moving from B to K; but thence
forward it rises from the center again by the same degrees, as it
approached by before.
15. IF (in fig. 77.) the line B C be perpendicular to A B; then it has
been observed above[89], that the centripetal power may be so balanced
with the progressive motion of the body, that the body may keep moving
round the center A constantly at the same distance; as a body does,
when whirled about any point, to which it is tyed by a string. If the
centripetal power be too weak to produce this effect, the motion of
the body will presently become oblique to the line drawn from itself
to the center, after the manner of the first of the two cases, which
we have been considering. If the centripetal power be stronger, than
what is required to carry the body in a circle, the motion of the body
will presently fall in with the second of the cases, we have been
considering.
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