Let me repeat this one more time. The potential energy changes in a system of two masses are analogous to the changes occurring within a system of unlike charges.

Both systems are composed of mutually attracting components. Work is required to separate the components, two masses or two unlike charges, and as they are moved further apart, the system gains potential energy equal to the work performed in moving the components of the system from the initial to the final position.

We are repeating this concept more than a few times because it really is important. If I had not worked with a large number of premedical students, I might think this is enough, but I know you need to hear it.

Think of two masses attracted by gravity or two unlike charges attracted by electric force as having potential energy down below zero, a negative number. How far below zero is the potential energy? That tells you how much work needs to be done to completely separate the components of the system, the binding energy.
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