If the cord of one of the balls should be cut, it would fall to the earth, for the reason that the attractive force of the great mass of the earth is so much greater than the force of attraction in its companion ball.
INDESTRUCTIBILITY OF GRAVITATION.—Gravity cannot be produced or destroyed. It acts between all parts of bodies equally; the force being proportioned to their mass. It is not affected by any intervening substance; and is transmitted instantaneously, whatever the distance may be.
While, therefore, it is impossible to divest matter of this property, there are two conditions which neutralize its effect. The first of these is position. Let us take two balls, one solid and the other hollow, but of the same mass, or density. If the cavity of the one is large enough to receive the other, it is obvious that while gravity is still present the lines of attraction being equal at all points, and radially, there can be no pull which moves them together.
DISTANCE REDUCES GRAVITATIONAL PULL.—Or the balls may be such distance apart that the attractive force ceases. At the center of the earth an object would not weigh anything. A pound of iron and an ounce of wood, one sixteen times the mass of the other, would be the same,—absolutely without weight.
If the object should be far away in space it would not be influenced by the earth's gravity; so it will be understood that position plays an important part in the attraction of mass for mass.
HOW MOTION ANTAGONIZES GRAVITY.—The second way to neutralize gravity, is by motion. A ball thrown upwardly, antagonizes the force of gravity during the period of its ascent. In like manner, when an object is projected horizontally, while its mass is still the same, its weight is less.
Motion is that which is constantly combating the action of gravity. A body moving in a circle must be acted upon by two forces, one which tends to draw it inwardly, and the other which seeks to throw it outwardly.
The former is called centripetal, and the latter centrifugal motion. Gravity, therefore, represents centripetal, and motion centrifugal force.
If the rotative speed of the earth should be retarded, all objects on the earth would be increased in weight, and if the motion should be accelerated objects would become lighter, and if sufficient speed should be attained all matter would fly off the surface, just as dirt dies off the rim of a wheel at certain speeds.
A TANGENT.—When an object is thrown horizontally the line of flight is tangential to the earth, or at right angles to the force of gravity. Such a course in a flying machine finds less resistance than if it should be projected upwardly, or directly opposite the centripetal pull.