• Tags: nuclear-force, strong, short-range
• Difficulty: Foundation

The nuclear force (strong force) binds nucleons together against the electrostatic repulsion between protons. Key properties: (1) Strongest known force — ~100 times stronger than electromagnetic force at nuclear distances. (2) Short-range: effective only up to ~3 fm; negligible beyond that. (3) Charge-independent: p-p, n-n, and p-n nuclear forces are approximately equal (charge independence). (4) Saturating: each nucleon interacts only with its nearest neighbors, not all other nucleons. This explains why BE/A is roughly constant rather than proportional to A. (5) Attractive at distances > 0.8 fm but strongly repulsive at shorter distances (hard core), preventing nucleons from overlapping. (6) Spin-dependent: the force is stronger when nucleon spins are parallel. The nuclear force cannot be explained by classical physics — it's mediated by pion exchange (Yukawa's meson theory, 1935). The exchange of virtual pions between nucleons creates the attractive potential.