As the two protons get closer, the repulsive electromagnetic force between them increases. This force is due to the positive charges of the protons, which repel each other. The closer the protons get, the stronger the repulsive force becomes.

As the repulsive force increases, it causes the protons to accelerate away from each other. This means that the protons will move faster and faster as they get closer together.

The repulsive force between the protons is so strong that it will eventually prevent them from getting any closer. This means that the protons will reach a point where they are at a fixed distance from each other and will not be able to move any closer.

The distance between the two protons at which they reach equilibrium is called the "equilibrium separation distance". This distance is determined by the balance between the repulsive electromagnetic force and the attractive nuclear force.

At the equilibrium separation distance, the repulsive electromagnetic force and the attractive nuclear force are equal in strength. This means that the protons are not moving any closer together or moving away from each other. They are in a state of equilibrium.