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The strong nuclear force holds protons and neutrons together in nuclei, overcoming electrostatic repulsion among protons. It is the strongest known force but acts only at extremely short range (~1 fm). The strong force is charge-independent (protons and neutrons feel it equally) and exhibits saturation: binding energy per nucleon saturates (~8.8 MeV) as nuclei grow, indicating each nucleon binds primarily to its neighbors rather than all other nucleons.
From your study of nuclear structure, you know that nuclei contain positively charged protons packed within a radius of a few femtometers. The electrostatic repulsion between protons at that range is enormous — on the order of hundreds of keV per proton pair. Yet nuclei are stable. Something must be overpowering electrostatic repulsion, and that something is the strong nuclear force, sometimes called the nuclear force or hadronic force.
The defining feature of the strong force is its extreme short range. Unlike gravity or electrostatics, which fall off as 1/r² and extend to infinity, the strong force drops to essentially zero beyond about 2–3 fm (~2–3 × 10⁻¹⁵ m). This is why only nearby nucleons interact — a proton in a large nucleus does not feel a direct strong-force pull from protons on the other side. This behavior is well modeled by a Yukawa potential: V(r) ∝ (e−r/r₀)/r, where r₀ ≈ 1.4 fm is the range. At short range (< 0.5 fm) the force becomes repulsive, giving nucleons a hard core that prevents nuclei from collapsing inward.
Charge independence is the key empirical observation that the strong force is nearly identical between proton-proton, proton-neutron, and neutron-neutron pairs. This symmetry hints at a deeper underlying structure — protons and neutrons are both nucleons, different charge states of the same particle in the modern quark picture. The strong force between nucleons is actually a residual effect of the color force binding quarks inside each nucleon, analogous to how van der Waals forces between neutral molecules are residuals of the underlying electromagnetic interaction.
Saturation is the practical consequence of short range. Because each nucleon binds only to its immediate neighbors, the total binding energy scales roughly linearly with the number of nucleons A. Binding energy per nucleon B/A peaks near iron (≈ 8.8 MeV/nucleon) and stays roughly flat across medium and heavy nuclei. If the strong force were long-range like gravity, B/A would keep increasing with A and matter would not have stable, finite nuclei — everything would clump together. Saturation is why nuclei have well-defined densities (~2.3 × 10¹⁷ kg/m³) and roughly constant density cores: adding more nucleons grows the nucleus but does not densify its core.