Title Isaac Newton's Principia, English Translation English 36.2 MB 594
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MATH.-STAT.

Page 297

SEC. IY.1 OF NATURAL PHILOSOPHY. 291

COR. 9. And although these motions becoming eccentrical should be

performed in spirals approaching to an oval figure, yet, conceiving the

several revolutions of those spirals to be at the same distances from each

other, and to approach to the centre by the same degrees as the spiral above

described, we may also understand how the motions of bodies may be per
formed in spirals of that kind.

PROPOSITION XVI. THEOREM XIII.
If the density of the medium in each of the places be reciprocally as the

distance of the &amp;gt;, placesfrom the immoveable centre, and the centripetal
force be reciprocally as any power of the same distance, I say, that the

body may revolve in a spiral intersecting all the radii drawn from
that centre in a given, angle.

This is demonstrated in the same manner as

the foregoing Proposition. For if the centri

petal force in P be reciprocally as any power
SPn+ 1 of the distance SP whose index is n
+ 1

;
it will be collected, as above, that the

time in which the body describes any arc PQ,
i

will be as PQ, X PS 2U
;
and the resistance in

i!!
x _

n;
raS

&quot;~X SP n; PQ, X SP&quot;XSQ,

, , 1 in X OS . 1 \n X OS .
therefore as

Qp&quot;^~gpirqTT
tliat 1S

&amp;gt; (
because -

~~Qp~~
1S a lven

quantity), reciprocally as SP n+ ! . And therefore, since the velocity is recip

rocally as SP
3

&quot;,

the density in P will be reciprocally as SP.

COR. 1. The resistance is to the centripetal force as 1 ^//. X OS
to OP.

COR. 2. If the centripetal force be reciprocally as SP
3

. 1 w will be

===
;
and therefore the resistance and density of the medium will be

nothing, as in Prop. IX, Book I.

COR. 3. If the centripetal force be reciprocally as any power of the ra

dius SP, whose index is greater than the number 3, the affirmative resist

ance will be changed into a negative.

SCHOLIUM.

This Proposition and the former, which relate to mediums of unequal

density, are to be understood of the motion of bodies that are so small, that

the greater density of the medium on one side of the body above that on

the other is not to be considered. I suppose also the resistance, cateris

paribus, to be proportional to its density. Whence, in mediums whose

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292 THE MATHEMATICAL PRINCIPLES
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BoOK II

force of resistance is not as the density, the density must be so much aug
mented or diminished, that either the excess of the resistance may be taken

away, or the defect supplied.

PROPOSITION XVII. PROBLEM IV
Tofind the centripetalforce and the resisting force of the medium, by
which a body, the law of the velocity being given, shall revolve in a

given spiral.
Let that spiral be PQR. From the velocity,

with which the body goes over the very small arc

PQ,, the time will be given : and from the altitude

TQ,, which is as the centripetal force, and the

square of the time, that force will be given. Then

from the difference RSr of the areas PSQ, and
Q,SR described in equal particles of time, the re

tardation of the body will be given ; and from

the retardation will be found the resisting force

and density of the medium.

PROPOSITION XVIII. PROBLEM V.
The law of centripptalforce being given, to find the density of the me
dium in each of the places thereof, by which a body may describe a

given spiral.
From the centripetal force the velocity in each place must be found ;

then from the retardation of the velocity the density of the medium is

found, as in the foregoing Proposition.

But I have explained the method of managing these Problems in the

tenth Proposition and second Lemma of this Book; and will no longer
detain the reader in these perplexed disquisitions. I shall now add some

things relating to the forces of progressive bodies,
and to the density and

resistance of those mediums in which the motions hitherto treated of, and

those akin to them, are performed.

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