So during a particle physics lecture, the lecturer used heisenberg's uncertainty principle to set a lower limit on the KE of the quarks bound in a proton (given the mass,size of proton and the mass of u/d quarks), which is of the order of 100 MeV, while the mass of quarks is about several MeV/c^2. Then, he said given the small size and low mass of quarks, it is "unnatural" for them to be composite particles, and continued to elaborate. I failed to listen to the whole explanation and all I heard was "as it would require a fine-tuned balance between kinetic energy and potential energy". I didn't get the chance to ask him again and now I don't exactly know what that last statement would mean. So I started to ponder for hours what that has to do with composite particles. I know that a bound system of particles need to have negative total energy (KE+PE, at least in classical mechanics). Suppose quarks are indeed composed of smaller particles. As far as I can tell, given the small size of quarks, these particles would have even larger minimum KE than the quarks do (>100 Mev) and the PE has to be very negative to make the total energy scale back to several MeV. I still fail to appreciate why KE and PE have to be balanced. I mean, the system is bound as long as the net energy is negative isn't it?