pions also have nonzero rest masses. However, those masses are almost an order of magnitude smaller than that of the nucleons, roughly 45 MeV, where are the relevant current quark masses, around .

The pion is one of the particles that mediate the residual strong interaction between a pair of nucleons. This interaction is attractive: it pulls the nucleons together. Written in a non-relativistic form, it is called the Yukawa potential.