Newton's First Law is the Law of Inertia. Newton generalized the ideas of Galileo Galilei, who asserted that an object will continue to move indefinitely in the absence of forces. Let us first state the first law and then discuss the conceptual questions a little further:
The First Law In the absence of an unbalanced applied force, a body at rest remains at rest and a body already in motion remains in motion with a constant velocity.
This observation is seemingly contrary to our everyday experience in which objects tend to slow down on their own. If you want an example for this, just think about what is happening when you push a heavy piece of furniture, say a couch, around: You need to use force constantly to keep the couch moving. As soon as you stop exerting a force on the couch, it stops. This was exactly what you were expecting from the prevailing Aristotelian, per-Newton/Galileo, view of the world, which stated that you need to provide a constant force in order to keep an object moving.
But what really slows the couch down when you stop pushing is not the absence of a force, but instead the presence of a force, the friction force between the couch and the carpet.
Thus you can begin to get an idea of what a conceptual leap was performed by Newton and Galileo. The Aristotelian view of force and motion was deeply ingrained in their contemporaries in the 17th century and indeed still deeply influences our conceptions about these issues.
What is "Inertia"? Inertia is the natural tendency of an object to maintain a state of rest or remain in uniform motion in a straight line. The concept of mass is central to the understanding of inertia.
© MultiMedia Physics, 1999