ZingPath: Newton's First Law

The concept of motion has evolved over the past two thousand years, and much of this change occurred as a result of the work done by Galileo and Newton.

Galileo could observe the effects of friction on the motion of objects, and sought to remove this force entirely. But, he could not entirely neglect the effects of friction, and so had to conduct thought experiments instead. He chose to set an object in motion by releasing it on an inclined plane. He noticed that when the amount of friction was small, an object would move down one plane, and reach a final height that was almost the same as the initial height. The object's speed would decrease as it moved up the second plane until it got back to its initial height. Galileo then concluded that in the absence of friction and a second plane, the object would continue moving with a constant velocity forever, because it would never reach its initial height. It is important to focus on the fact that an object will not change its velocity in the absence of a force, and it does not need a force in order to continue its motion as previously thought.

Newton later used Galileo's concept of motion to construct his own laws. In fact, Newton's 1st Law is often called the Law of Inertia. Inertia is simply the resistance to a change in an object's motion, whether it is at rest or moving. In other words, the more massive the object, the greater its resistance to a change in its state of rest or motion.