A nucleus holds many protons together despite their mutual repulsion. Which of these statements about the force responsible are correct?

• I. It is much stronger than the Coulomb force at distances of a few femtometres.
• II. It acts almost equally between proton–proton, neutron–neutron and proton–neutron pairs.
• III. It falls off as the inverse square of distance, like gravity.
• IV. It becomes strongly repulsive when nucleons come closer than about 0.8 fm.