For 2000 years, scientists and philosophers thought that the “natural state” of an object is to be at rest. An object at rest requires no explanation. A moving object, though, is not in its natural state and thus requires an explanation: Why is this object moving? What keeps it going?

Galileo, in around 1600, was one of the first scientists to carry out controlled experiments. Many careful measurements in which he minimized the influence of friction led Galileo to conclude that in the absence of friction or air resistance, a moving object would continue to move along a straight line forever with no loss of speed. In other words, the natural state of an object—its behavior if free of external influences—is not rest but is uniform motion with constant velocity! “At rest” has no special significance in Galileo’s view of motion; it is simply uniform motion that happens to have $\vec{v}=\vec{0}$.

It was left to Newton to generalize this result, and today we call it Newton’s first law of motion.

Newton’s first law is also known as the law of inertia. If an object is at rest, it has a tendency to stay at rest. If it is moving, it has a tendency to continue moving with the same velocity.

Notice the “if and only if” aspect of Newton’s first law. If an object is at rest or moves with constant velocity, then we can conclude that there is no net force acting on it. Conversely, if no net force is acting on it, we can conclude that the object will have constant velocity, not just constant speed. The direction remains constant, too!

An object on which the net force is zero—and thus is either at rest or moving in a straight line with constant velocity—is said to be in mechanical equilibrium. As FIGURE 5.19 shows, objects in mechanical equilibrium have no acceleration: $\vec{a}=\vec{0}$.

FIGURE 5.19 Two examples of mechanical equilibrium.