So what causes an object to move? Newton’s first law says no cause is needed for an object to move! Uniform motion is the object’s natural state. Nothing at all is required for it to remain in that state. The proper question, according to Newton, is: What causes an object to change its velocity? Newton, with Galileo’s help, also gave us the answer. A force is what causes an object to change its velocity.
The preceding paragraph contains the essence of Newtonian mechanics. This new perspective on motion, however, is often contrary to our common experience. We all know perfectly well that you must keep pushing an object—exerting a force on it—to keep it moving. Newton is asking us to change our point of view and to consider motion from the object’s perspective rather than from our personal perspective. As far as the object is concerned, our push is just one of several forces acting on it. Others might include friction, air resistance, or gravity. Only by knowing the net force can we determine the object’s motion.
Newton’s first law may seem to be merely a special case of Newton’s second law. After all, the equation tells us that an object moving with constant velocity has . The difficulty is that the second law assumes that we already know what force is. The purpose of the first law is to identify a force as something that disturbs a state of equilibrium. The second law then describes how the object responds to this force. Thus from a logical perspective, the first law really is a separate statement that must precede the second law. But this is a rather formal distinction. From a pedagogical perspective it is better—as we have done—to use a commonsense understanding of force and start with Newton’s second law.