In short, every-day experience gives us illustrations of what might be called compound motion, which makes it seem altogether plausible that, if the earth is in motion, objects at its surface will partake of that motion in a way that does not interfere with any other movements to which they may be subjected.

As the Copernican doctrine made its way, this idea of compound motion naturally received more and more attention, and such experiments as those of Galileo prepared the way for a new interpretation of the mechanical principles involved.
The great difficulty was that the subject of moving bodies had all along been contemplated from a wrong point of view.

Since force must be applied to an object to put it in motion, it was perhaps not unnaturally assumed that similar force must continue to be applied to keep the object in motion.

When, for example, a stone is thrown from the hand, the direct force applied necessarily ceases as soon as the projectile leaves the hand.

The stone, nevertheless, flies on for a certain distance and then falls to the ground.