One of the challenging ideas of using energy at the atomic and the molecular level is the idea of bound states. What this means is that you might start with two objects that have essentially zero kinetic energy, they get close and interact strongly in an attractive way. They find some way to emit energy into another form and wind up being stuck together -- bound. You have to put energy in -- do work on them -- in order to get them apart. Let's work through the language of potential energy to see how to talk about this. (Since we are making a model that looks like atoms and molecules, we will ignore friction.) Let's first talk about a simple problem that you have now had some practice with -- the motion of a skateboarder on a track. Suppose the track looks like a dip in the ground as shown in the figure. A potential shape like this is often referred to as a potential well, since it looks like a dug-out area for a well