Question 101: Gravitation

Question

Two planets A and B of radii $$\mathrm{R}$$ and 1.5 R have densities $$\rho$$ and $$\rho / 2$$ respectively. The ratio of acceleration due to gravity at the surface of $$\mathrm{B}$$ to $$\mathrm{A}$$ is:

Accepted Answer

The acceleration due to gravity at the surface of a planet is given by the formula:
$$g=\frac{GM}{R^2}$$
where $$G$$ is the gravitational constant, $$M$$ is the mass of the planet, and $$R$$ is the radius of the planet.
In this problem, we are given two planets A and B, with radii $$R_A$$ and $$R_B=1.5R_A$$, and densities $$\rho_A$$ and $$\rho_B=\frac{\rho_A}{2}$$, respectively. We can use the formula above to calculate the acceleration due to gravity at the surface of each planet:
$$g_A=\frac{GM_A}{R_A^2}=\frac{4}{3}\pi G\rho_AR_A$$
and

$$g_B=\frac{GM_B}{R_B^2}=\frac{4}{3}\pi G\rho_B R_B=\frac{4}{3}\pi G\frac{\rho_A}{2}1.5R_A=\frac{3}{2}g_A$$
To find the ratio of the accelerations due to gravity at the surfaces of planets A and B, we can divide the expression for $$g_B$$ by the expression for $$g_A$$:
$$\frac{g_B}{g_A}=\frac{\frac{3}{2}g_A}{g_A}=\frac{3}{2}$$
So, we can see that the acceleration due to gravity at the surface of planet B is $$\frac{3}{2}$$ times larger than that at the surface of planet A.
However, the question asks for the ratio of the gravitational accelerations at the surfaces of the planets, which is equal to the ratio of the densities times the ratio of the radii:
$$\frac{g_B}{g_A}=\frac{\rho_B}{\rho_A}\cdot\frac{R_B}{R_A}=\frac{1/2}{1}\cdot\frac{1.5R_A}{R_A}=\frac{3}{4}$$
Therefore, the ratio of the gravitational accelerations at the surfaces of the planets is $$3/4$$, which is our final answer.

Answer

2 : 1

Answer

2 : 3

Answer

4 : 3

Answer

3 : 4

Gravitation

Two planets A and B of radii $\mathrm{R} and 1.5 R have densities \rho and \rho / 2 respectively. The ratio of acceleration due to gravity at the surface of \mathrm{B} to \mathrm{A}$ is:

Choose the correct answer: