Physical World

9 × 10¹³ J

Step 1: Given Δm = 1 g = 0.001 kg = 10⁻³ kg, c = 3 × 10⁸ m/s. Step 2: Formula: ΔE = (Δm) × c². Step 3: c² = (3 × 10⁸)² = 9 × 10¹⁶ m²/s². Step 4: ΔE = 10⁻³ × 9 × 10¹⁶ = 9 × 10¹³ J. Step 5: This enormous energy from just 1 gram shows why nuclear reactions release such tremendous energy. Option B (3 × 10¹³) is wrong — a common error of using c instead of c². Option C forgets to square 10⁸ correctly.

Gravitational < Weak < Electromagnetic < Strong

Step 1: The four fundamental forces arranged by relative strength from weakest to strongest are: Gravitational, Weak, Electromagnetic, and Strong (Nuclear). Step 2: Their relative magnitudes are in the ratio 1 : 10²⁶ : 10³⁶ : 10³⁹. Step 3: Gravitational force is the weakest — it cannot even hold a proton and neutron together in the nucleus. Step 4: Strong nuclear force is the strongest, binding protons and neutrons inside the tiny nucleus despite the electromagnetic repulsion between protons. Step 5: Options B, C, and D all incorrectly place weak force below gravitational or rearrange electrom

π-meson (pion)

Step 1: Every fundamental force has an associated field particle (also called exchange particle or mediator) that explains how two bodies interact from a distance. Step 2: Strong Nuclear Force → π-meson (pion), proposed by H. Yukawa. Step 3: Gravitational Force → Graviton (theoretical, not yet detected). Step 4: Electromagnetic Force → Photon. Step 5: Weak Force → Neutrino (or W and Z bosons in modern physics). Option A (Graviton) is for gravity. Option B (Neutrino) is for weak force. Option D (Positron) is not a field particle — it is the antiparticle of an electron.

Conservation of Number of Nucleons (Mass Number)

Step 1: In the given decay, ₉₂U²³⁸ → ₉₀Th²³⁴ + ₂He⁴. Step 2: Mass number of reactant = 238. Mass numbers of products = 234 + 4 = 238. They are equal. Step 3: This is governed by the Conservation of Number of Nucleons (also called Conservation of Mass Number). Step 4: Note that charge (atomic number) is also conserved: 92 = 90 + 2 — but the question specifically asks about the total nucleon count (238). Step 5: Conservation of charge and conservation of nucleons are both satisfied here, but the direct law for nucleon count is option C. Option A (charge) refers to atomic number conservation, not