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AXIOMS.

13. EVERY body naturally endeavours to continue in its present state, whether it be at rest, or moving uniformly in a right line.

14. The Change or Alteration of Motion, by any external force, is always proportional to that force, and in the direction of the right line in which it acts.

15. Action and Re-action, between any two bodies, are equal and contrary. That is, by Action and Re-action, equal changes of motion are produced in bodies acting on each other; and these changes are directed towards opposite or contrary parts.

GENERAL LAWS OF MOTION, &c.

PROPOSITION I

16. The Quantity of Matter, in all Bodies, is in the Compound Ratio of their Magnitudes and Densities.

THAT is, b is as md; where b denotes the body or quantity of matter, m its magnitude, and d its density.

For, by art. 4, in bodies of equal magnitude, the mass or quantity of matter is as the density. But, the densities remaining, the mass is as the magnitude: that is, a double magnitude contains a double quantity of matter, a triple magnitude a triple quantity, and so on. Therefore the mass is in the compound ratio of the magnitude and density.

17. Corol. 1. In similar bodies, the masses are as the densities and cubes of the diameters, or of any like linear dimensions. For the magnitudes of bodies are as the cubes of the diameters, &c.

18. Corol. 2. The masses are as the magnitudes and specific gravities. For, by art. 4 and 12, the densities of bodies are as the specific gravities.

19. Scholium. Hence, if b denote any body, or the quantity of matter in it, m its magnitude, d its density, g its

specific

specific gravity, and a its diameter or other dimension; then, a (pronounced or named as) being the mark for general proportion, from this proposition and its corollaries we have these general proportions :

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20. The Momentum, or Quantity of Motion, generated by a Single Impulse, or any Momentary Force, is as the Generating Force.

THAT is, m is as f; where m denotes the momentum, and fthe force.

So

For every effect is proportional to its adequate cause. that a double force will impress a double quantity of motion; a triple force, a triple motion; and so on. That is, the motion impressed, is as the motive force which produces it.

PROPOSITION III.

21. The Momenta, or Quantities of Motion, in moving Bodies, are in the Compound Ratio of the Masses and Velocities.

That is, m is as bv.

FOR, the motion of any body being made up of the motions of all its parts, if the velocities be equal, the momenta will be as the masses; for a double mass will strike with a double force; a triple mass, with a triple force, and so on. Again, when the mass is the same, it will require a double force to move it with a double velocity, a triple force with a triple velocity, and so on; that is, the motive force is as the velocity; but the momentum impressed, is as the force which produces it, by prop. 2; and therefore the momentum is as the velocity when the mass is the same. But the momentum was found to be as the mass when the velocity is the same, Consequently,

Consequently, when neither are the same, the momentum is in the compound ratio of both the mass and velocity.

PROPOSITION IV.

22. In Uniform Motions, the Spaces described are in the Compound Ratio of the Velocities and the Times of their Description.

That is, s is as tv.

FOR, by the nature of uniform motion, the greater the velocity, the greater is the space described in any one and the same time; that is, the space is as the velocity, when the times are equal. And when the velocity is the same, the space will be as the time; that is, in a double time a double space will be described; in a triple time, a triple space; and so on. Therefore universally, the space is in the compound ratio of the velocity and the time of description.

23. Corol. 1. In uniform motions, the time is as the space directly, and velocity reciprocally; or as the space divided by the velocity. And when the velocity is the same, the time is as the space. But when the space is the same, the time is reciprocally as the velocity.

24. Corol. 2. The velocity is as the space directly and the time reciprocally; or as the space divided by the time. And when the time is the same, the velocity is as the space. But when the space is the same, the velocity is reciprocally as the time.

Scholium.

25. In uniform motions generated by momentary impulse, let b = any body or quantity of matter to be moved,

f force of impulse acting on the body b,

m

the uniform velocity generated in 6,

the momentum generated in b,

= the space described by the body b,

?= the time of describing the spaces with the veloc. v.

Then from the last three propositions and corollaries, we have these three general proportions, namely, fa m, m œ bʊ, and so to; from which is derived the following table of the general relations of those six quantities, in uniform motions and impulsive or percussive forces:

VOL. II.

Q

f a m

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By means of which, may be resolved all questions relating to uniform motions, and the effects of momentary or impulsive forces.

PROPOSITION V.

26. The Momentum generated by a Constant and Uniform Force acting for any Time, is in the Compound Ratio of the Force and Time of Acting.

That is, m is as ft.

For, supposing the time divided into very small parts, by prop. 2, the momentum in each particle of time is the same, and therefore the whole momentum will be as the whole time, or sum of all the small parts. But by the same prop. the momentum for each small time, is also as the motive force. Consequently the whole momentum generated, is in the compound ratio of the force and time of acting.

27. Corol. 1. The motion, or momentum, lost or destroyed in any time, is also in the compound ratio of the force and time. For whatever momentum any force generates in a given time; the same momentum will an equal force destroy in the same or equal time; acting in a contrary direction.

And the same is true of the increase or decrease of motion, by forces that conspire with, or oppose the motion of bodies.

28. Corol. 2. The velocity generated, or destroyed, in any time, is directly as the force and time, and reciprocally as the body or mass of matter.-For, by this and the 3d prop. the compound ratio of the body and velocity, is as that of the force and time; and therefore the velocity is as the force and time divided by the body. And if the body and force be given, or constant, the velocity will be as the time.

PROPOSITION

PROPOSITION VI.

29. The Spacespassed over by Bodies, urged by any Constant and Uniform Forces,acting during any Times, are in the compound Ratio of the Forces and Squares of the Times directly, and the Body or Mass reciprocally.

Or, the Spaces are as the Squares of the Times, when the Force and Body are given.

THAT is, & is as√12,,

8

or as 2 when ƒ and 6 are given. For, let v denote the velocity acquired at the end of any time, by any given body b, when it has passed over the space 8. Then, because the velocity is as the time, by the last corol. therefore v is the velocity att, or at the middle point of the time; and as the increase of velocity is uniform, the same spaces will be described in the same time, by the velocityv, uniformly continued from beginning to end. But, in uniform motions, the space is in the compound ratio of the time and velocity; therefore s is as tv, or indeed 8 — tv. But, by the last corol. the velocity v is as

fiz

ft

, or as the force and time directly, and as the body reciprocally. Therefore 8, or 1⁄2 tv, is as -; that is, the space is as the force and square of the time directly, and as the body reciprocally. Ora is as 12, the square of the time only, when 6 and ƒ are given.

30. Corol. 1. The space is also as tv, or in the compound ratio of the time and velocity; and f being given. For, &= tv is the space actually described. But tv is the space which might be described in the same time t, with the last velocity v, if it were uniformly continued for the same or an equal time. Therefore the space s, or tv, which is actually described, is just half the space tv, which would be described, with the last or greatest velocity, uniformly continued for an equal time t.

81. Corol. 2. The space & is also as v2, the square of the velocity; because the velocity v is as the time t.

Scholium.

32. Propositions 3, 4, 5, 6, give theorems for resolving all questions relating to motions uniformly accelerated. Thus,

put

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