Q122 / 150 · MHT CET 2025 (23 Apr Shift 2) · Physics

The gravitational pull of the moon is $\left(\frac{1}{6}\right)^{\text {th }}$ of the earth and mass of moon is $\left(\frac{1}{8}\right)^{\text {th }}$ of the earth. This implies that the

  • A
    radius of moon is $(1 / 4)^{\text {th }}$ of the earth's radius.
  • Correct answer
    B
    radius of the earth is $(\sqrt{4 / 3})^{\text {th }}$ of the moon's radius.
  • C
    moon's radius is half that of the earth.
  • D
    radius of the earth is $(4 / 3)^{\text {th }}$ of the moon's radius.

Solution

The acceleration due to gravity at a celestial body's surface is governed by $g = \frac{GM}{R^2}$, where $M$ is mass and $R$ is radius. Given that lunar gravity is one-sixth of Earth's, $g_m = \frac{1}{6} g_e$, and lunar mass is one-eighth of Earth's, $M_m = \frac{1}{8} M_e$.

Substituting into the gravity formula yields $\frac{GM_m}{R_m^2} = \frac{1}{6} \cdot \frac{GM_e}{R_e^2}$. After canceling $G$ and $M_e$ using the mass relation, $\frac{1}{8R_m^2} = \frac{1}{6R_e^2}$.

Cross-multiplying gives $6R_e^2 = 8R_m^2$, simplifying to $3R_e^2 = 4R_m^2$. Solving for the radii, $R_e = \sqrt{\frac{4}{3}} R_m$, confirming that Earth's radius is $\sqrt{4/3}$ times the Moon's.

Only option B matches this result.

Hence, the correct answer is option: 'B'.

Relevant Questions