Force of Gravity

The force by virtue of which all terrestrial bodies fall to the earth when unsupported. As a terrestrial force it may be considered constant for the same place, but as it is practically the resultant of the earth’s attraction and the centrifugal force arising from its rotation, and as the earth is neither homogeneous nor a perfect sphere, it will vary slightly with the latitude, being greatest at the poles and least on the equator, and it will also vary in an insignificant degree from place to place in the same latitude. Gravity is distinguished in dynamics as the only constant force with which we have to do. It differs also from all others in this, that its measure is independent of ma . Other forces are measured by the product of the ma moved into the velocity imparted in the unit of time; but as gravity impre es the same velocity upon all ma es, great or small, ma is properly omitted in its measure. The velocity impre ed by it during each second of its action, or the acceleration , is about 32.1808 feet in latitude 45°, about 32.0977 at the equator, and 32.2629 at the poles. This number in gunnery is indicated by the algebraic symbol g . Its exact value at any place is best determined by the length of the simple second’s pendulum at that place. The value for g for bodies falling in the air is very nearly true for dense substances presenting small surfaces, when the fall is limited by a few seconds. For the ordinary time of flight of projectiles it can be used without material error.