The first law of motion teaches us that a body in
motion, if acted on by no force, will move in a straight line.
Hence, if we see a body moving in a curve, we know that it is
acted on by a force in the direction towards which the motion
curves. A familiar example is that of a stone thrown from the
hand. If the stone were not attracted by the earth, it would go on
forever in the line of throw, and leave the earth entirely. But
under the attraction of the earth, it is drawn down and down, as
it travels onward, until finally it reaches the ground. The faster
the stone is thrown, of course, the farther it will go, and the
greater will be the sweep of the curve of its path. If it were a
cannon-ball, the first part of the curve would be nearly a right
line. If we could fire a cannon-ball horizontally from the top of
a high mountain with a velocity of five miles a second, and if it
were not resisted by the air, the curvature of the path would be
equal to that of the surface of our earth, and so the ball would
never reach the earth, but would revolve round it like a little
satellite in an orbit of its own. Could this be done, the
astronomer would be able, knowing the velocity of the ball, to
calculate the attraction of the earth as well as we determine it
by actually observing the motion of falling bodies around us.
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