Force Fields and Potential Energy

One reason Newton’s discoveries were so remarkable is that they explained the underlying basis for many phenomena that hadn’t previously been understood. Before Newton, scientists could explain how the planets moved about the sun, but they could not explain “why.” Newton’s gravity provided the answer.

Or did it? We can say Earth’s gravity holds the moon in its orbit, but just naming it does little to explain what’s actually happening. To truly explain why the force of gravity does what it does, requires a few more modern concepts.

The first of these concepts is potential energy. This is a type of energy that the moon possesses simply by virtue of its proximity relative to Earth. Potential energy can be thought of as stored energy that has the potential to cause motion. The moon isn’t the only thing that has potential energy. Any massive object near Earth has it—even that cup of coffee sitting on the table beside you. If you were to accidentally knock it off of the table, the stored energy would spring into action, causing the cup to move downward toward the floor, where it’s bound to make a mess.

An alternative way to explain this connection is through the use of something called a gravitational field, essentially a region of influence that surrounds a massive body and quantifies its gravitational attraction at every point around it.

Consider two massive objects, say, Tweedledee and Tweedledum. If Tweedledum stands in a certain location near Tweedledee, the gravitational attraction he feels is determined by the strength of his brother’s gravitational field at that location. Even though Tweedledee has a finite size, his gravitational field fills all space around him—and infinitely far into the distance. The strength of the field decreases with distance. Near to him, the gravitational field is large, while far away it is small.

The concepts of potential energy and fields turn out to be useful not only for interpreting the workings of gravity, but also for understanding all forces of nature. We will revisit the relationship between forces, potential energy, and fields frequently.