Q4. Find the Coulomb force between two protons 2.0 fm apart.
F = ke2/r2 = 9.0 × 109 × (1.6 × 10−19)2/(2.0 × 10−15)2 ≈ 58 N, a huge force on a particle of mass 1.67 × 10−27 kg, which the nuclear force must overcome (reasoned from NCERT page 313).
Q5. Why do heavy stable nuclei have more neutrons than protons?
Neutrons add nuclear attraction without adding Coulomb repulsion. Stability needs a neutron-to-proton ratio of about 1 : 1 for light nuclei, rising to about 3 : 2 for heavy ones, where more neutrons are needed to overcome the repulsion among the many protons (NCERT Points to ponder 8, page 320).
Q6. Gravity and the Coulomb force reach to any distance, falling as 1/r2. How does the nuclear force differ?
It is far stronger at nuclear distances but has a short range: it falls rapidly to zero beyond a few femtometres, and it has no simple formula (NCERT pages 313–314).
Q7. Assertion: the binding energy per nucleon is nearly the same for nuclei with 30 < A < 170. Reason: the nuclear force is short-ranged and saturates. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: each nucleon is bound only by its near neighbours, so its binding does not grow with the size of the nucleus (NCERT pages 312–313).