For a spherical planet to act like a "point mass", so that externally F = GmM/r², it A) must have spherical mass symmetry. B) must have a mass

Question

For a spherical planet to act like a "point mass", so that externally F = GmM/r², it A) must have spherical mass symmetry. B) must have a mass << r c² / (2G). C) can not be hollow inside. D) must have constant density throughout.

Quizplus · Accepted Answer

For a spherical planet to act like a "point mass" and for the gravitational force equation $F = \frac{GmM}{r^2}$ to apply, the key requirement is that the planet must have spherical mass symmetry. This allows all of its mass to be considered as if it were concentrated at a point at its center when calculating gravitational forces outside the planet. Choices B, C, and D are not necessary conditions for the planet to behave as a point mass in gravitational calculations.

For a Spherical Planet to Act Like a "Point Mass