When you release the ball on the right side of this track, it will not have enough energy to escape the “well” unless it is released from above the black line.

The photograph above shows a track in the shape of a rounded vee, on which a ball can roll. (The ball in the photograph is a two-inch-diameter polyurethane ball.) At the bottom of the track, the ball has minimum (gravitational) potential energy. Since the ball, as long as it is on the track, cannot travel any lower than the bottom, it is most convenient if we consider the ball at this point to have zero potential energy. If you place the ball at any other point on the track, it has potential energy greater than zero, and gravity causes it to roll down towards the bottom, acquiring kinetic energy until it reaches the bottom. At this point, all of its original potential energy has been converted to kinetic energy. The ball now continues up the opposite side of the track, losing kinetic energy as it goes, until all the kinetic energy it had at the bottom is converted to potential energy (as long as you did not release the ball from above the black line, on the right side of the track). At this point, the ball has risen up the other side of the track to a point almost as high as that from which you initially released it. Ideally, it should reach exactly the same height as that from which you released it, but frictional losses prevent this. Left to roll this way, the ball would roll up and down, from one side of the track to the other, until friction eventually stopped it. If you release the ball from above the black line, its initial potential energy exceeds the potential energy it would have if it were placed on the flat portion of the track, at left. When it reaches the bottom of the track, then, its kinetic energy exceeds this potential energy, so when it reaches the flat part of the track, it still has some kinetic energy, and it leaves the well. Also, if you pushed the ball down the track from some position lower than the black line, you could in this way give it enough excess kinetic energy that it could escape the well.

The potential well is a useful model. It can describe a variety of systems in which an object is constrained in a field, and in which energy put into the system is exchanged periodically between potential and kinetic energy. These can be classical systems, such as the ball on the track in this demonstration, or a ball on the hills and valleys of a golf course, for example.  Other examples are pendulums, or masses hung on springs, oscillating up and down. This model is perhaps most often used in connection with quantum-mechanical systems, such as charge carriers in a crystal lattice, or atoms bound within a molecule.