More than two thousand years ago, Chinese philosophers wrote about motion in a book called the Mo Ching. They wrote: "Motion stops due to an opposing force. If there is no opposing force, the motion will never stop."
In ancient Greece, Aristotle wrote that horizontal motion was "unnatural." He thought that a push or a pull was needed to start and to keep something in horizontal motion. He aiso thought heavy objects fall at a faster rate, or acceleration, than lighter objects do. Aristotle's ideas about motion were widely accepted for more than 1,900 years.
During sixteenth century, Galileo questioned the ideas of Aristotle. He reasoned that if two bricks of the same mass fall at the same rate, side-by-side, they ought to fall at the same rate even when cemented rogether. In 1589, according to an often-repeated, unverified story, Galileo did an experiment from the Leaning Tower in Pisa, Italy. Galileo is supposed to have dropped two cannonballs of different masses at the same time from the top of the tower. Both cannonballs reached the ground at about the same time. Whether he actually did the experiment or not, his reasoning was correct.
Galileo Galilei also did experiments that showed that an object moving horizontally continues to move at the same speed unless a force opposes it. Galileo's experiments confirmed the ideas stated thousands of years earlier in the Mo Ching book.
