As was shown in the previous section, the nucleus contains lots of positive charge in a small space. Anyone who knows even a little about electrostatics will realise that like charge repels like, and that this force of repulsion becomes stronger the closer the charged particles become. Clearly, a very strong force must be in place to counteract this strong repulsion.

This force, which is still not fully explained, is called the strong nuclear force, or the binding energy, given its role in binding the nucleus together.

Often, when one cannot fully explain a situation, it is convenient to have model which goes some of the way to show the principles and forces involved. The most common model for this situation is a simple water drop.

One can imagine a nucleus as a single water drop, incompressible, and also un-stretchable. Because of the surface tension of the water (binding energy), the drop forms into a sphere, thus minimising surface area, and hence the energy needed to keep the sphere together.

However, the particles within that sphere of water are repelling away from each other. (this is one flaw of the water drop model, as water particles do not really repel). Thus, to minimise the repulsive force, the particles would ideally form a larger volume, but this is prevented by the surface tension of the drop.

Thus, an optimum value is reached: a compromise between the particles repelling, and the surface tension binding them together. Like all systems, the nucleus or water drop will try to minimise the energy of the system until a minimum is reached, that is, the optimum spherical shape.