Question

Write any two features of nuclear forces.

Hint:

Remember: Nuclear forces are strong, short-range, and saturating. Writing any two standard properties is sufficient for full marks.

Answer 1

Concept: Nuclear forces are the forces that bind protons and neutrons (nucleons) inside the atomic nucleus. They are very different from gravitational and electromagnetic forces. Any two features of nuclear forces:

Short range: Nuclear forces act only over very small distances (\( \sim 1\text{–}2 \, \text{fm} \)). Beyond a few femtometres, they become negligible.
Very strong in nature: They are the strongest known forces in nature at short distances, overcoming the electrostatic repulsion between protons.
Other valid features (any two acceptable in exams):

Charge independent (similar for p–p, n–n, p–n interactions)
Saturation property (each nucleon interacts only with nearby nucleons)
Attractive at intermediate range and repulsive at very short distances

Answer 2

Nuclear forces, the forces that hold protons and neutrons together inside a nucleus, have several distinguishing features. Two important ones, apart from the short-range and strength properties usually mentioned first, are charge independence and saturation.

Charge independence:
The nuclear force between any two nucleons is approximately the same in strength, whether the pair is proton-proton, neutron-neutron, or proton-neutron, once the electrical repulsion between two protons is separately accounted for and subtracted out. This is confirmed by scattering experiments: proton-proton nuclear scattering data, after correcting for the extra Coulomb repulsion between the two positive charges, matches neutron-proton scattering data closely, showing the strong nuclear force itself does not depend on electric charge, unlike the electromagnetic force.

Saturation property:
A nucleon inside a large nucleus does not interact equally with every other nucleon in that nucleus, it only interacts appreciably with its nearest neighbours, because the force falls off so sharply beyond a couple of femtometres. This is why the binding energy per nucleon does not keep increasing as more nucleons are added, past a certain nucleus size it levels off to roughly a constant value, about 8 MeV per nucleon for mid-sized nuclei. If every nucleon attracted every other nucleon in the nucleus, binding energy per nucleon would keep rising with nucleus size instead of levelling off, which is why this behaviour is called saturation.

These two features, charge independence and saturation, are valid alongside the more commonly quoted short-range and very-strong nature of the nuclear force, and any two correctly stated features are accepted as a complete answer.