Why do protons repel each other inside atomic nuclei?

Inside an atomic nucleus, protons would fly apart if only electromagnetism acted on them. Their positive charges create a strong repulsive force, especially as the number of protons increases. The strong nuclear force, however, is much stronger than electromagnetism at distances around a femtometer, which is about the size of a nucleus. It binds protons and neutrons together, but only over very short ranges. This is why stable nuclei exist, yet very large nuclei become harder to hold together and are often radioactive.