Correspondence.

Progress, Volume VI, Issue 11, 1 September 1911, Page 800

Correspondence.

(To the Editor.)

Milton, July 23, 1911. In his notes on weight, Mr. Peter Ellis informs us that unless a body is free to move, or in the act of moving, it has no real weight. If he had used the terms inertia or momentum in place of weight he would have been correct. Newton showed that every particle of matter attracts every other particle of matter, and this force of attraction is called gravitation. “The weight of a body is simply a measure of the mutual attraction between the body and the earth.’ and for convenience all English speaking people use the same terms or tables. The mutual attraction between two bodies, whether close together or at an infinite distance apart, is always directly proportional to their masses and inversely proportional to the square of their distances. A body 4000 thousand miles above the surface of the earth will only weigh one-fourth as much as it would on the surface, which is 4000 miles from the centre. A body which weighed a ton on the earth would, if weighed with a spring balance on the planet Mars, only weigh, roughly speaking, half a ton, because Mars has only about half of the mass of the earth. If the masses of two bodies are known, their pull on each other can always be found, however far apart they are by dividing the sum of then masses by the square of the distance between their centres. Mr. Ellis further says: The earth, taken as a whole, can have no real weight except in the direction of its orbit, unless it is drawn by influence outside or inside or above or below that orbit. If he had said momentum instead of weight he would have been right. The Earth in its yearly motion is circling round the Sun at the rate of 90,000 miles an hour, and but for the attraction of the sun it would, owing to centrifugal force, fly off into space. As it is, the two forces balance at 92,000,000 miles, the distance the earth is from the sun. If the earth’s speed were accelerated it would move further out; if retarded it would come in towards the sun. As long as the Earth meets with no resistance it will keep up its present speed, owing to its momentum, but if it is gradually losing speed, owing to tidal friction, as many think, it will be gradually drawn into the Sun.— am, etc., R. W. PE ARSE. (To the Editor.) In answer to Mr. Pearse’s criticisms on my notes on weight, let me say: First, that “inertia” applys no more to rest, than to motion, it being simply the inherent quality of matter whereby it cannot rest itself whilst in motion nor move itself when at rest; an absolute impotency, and that momentum is not synonymous with inertia. Mr. Pearse says “The weight of a body is simply a measure of the mutual attraction between the body and the Earth, which is quite true, but attraction never acts

without resistance, and if that resistance or counter attraction or motion is sufficient to destroy the original attraction the weight due to that attraction ceases; if, however, the counter attraction is insufficient (when, of course, motion ensues), there will be weight, because of the axiom I have laid down: “Unless a body is free to move or in the act of moving it has no real weight.” Mr. Pearse further says:. “A body 4000 miles above the surface of the Earth will only weigh one-fourth as much as it would on the surface. This proves my contention and shows that weight is not “intrinsic” in a body. Fig. 1 will illustrate this. Curve (a) indicates the surface of the Earth, the converging lines (b) (b) indicate weight, lessening by distance, a weight of say 1000 tons on the surface of the Earth lessens to 250 tons at (c) which is 4000 miles away; now, continuing this reduction to the vanishing point (d) the weight ceases altogether. Another sketch (Fig. 2) shows the effect of counter attraction. At (a) the attractions balance, and the body (represented by a spot) has no weight, because it is “not free to move” either towards (b) or (c), if it occupied the position (d), “it would be free to move” towards (b). and if on the other side of the neutral line (a) at say (e) it would be “free to move” towards (c), and when “free to move” would “have weight.” Mr. Pearse speaks of weighing with a spring balance. I would observe that we cannot weigh a body by a spring balance or any other instrument “unless that body is free to move.” In speaking of the attraction of the Sun, tending to draw the Barth towards it, Mr. Pearse truly says that the centrifugal force balances the attraction of the Sun. Of course the Earth orbit is the resultant of these forces. Did not the centrifugal force balance the attraction of the Sun, the Earth would tend towards it, and have weight in that direction. As it is, it can only have weight in the direction of its orbit (because it “is free to move” only in that direction).

The following notes on the Law of Opposition may throw some further light on the matter: There is a Law of Opposition operating everywhere at all times and in all places. In the physical world, before we can understand the constitution of matter or motion, we must recognise this law, for if we allow that there is more than one atom of material in existence, the plurality of atoms necessitates opposing forces, for atoms must be mutually attracted and mutually repelled, in order to form masses. One force cannot exist alone, for immediately it acts, it must be overcoming another force, and it cannot exist before it acts, therefore force is not force until it is called into action. Supposing, for instance, that the force of cohesion could act without opposition, it would go on acting indefinitely, since there would be no other force to stop it, and the material subject to its action would become denser and denser ad infinitum. Take the gravitation of the Earth. If that power were not opposed, the Earth would die of its own cohesive pressure. Tt is utterly impossible to isolate a force, for immediately that were done (if it could be done) the force could not act against nothing. In a mass, when the attractive and repulsive forces balance each other, there can be no motion, because the contrary forces destroy each other; force cannot be quiescent. When, however, forces are not equally opposed, motion on the side of the weaker force makes up the balance, the rate and amount of motion being, proportionate to the difference between the forces. Thus matter is subject to everlasting movement, and there is no rest in the universe, but a perenial rejuvenation by motion. —I am, etc., PETER ELLIS.