일반물리학 1 — Gravitation: Problem 66

66 Assume a planet is a uniform sphere of radius $R$ that (somehow) has a narrow radial tunnel through its center (Fig. 13-7). Also assume we can position an apple anywhere along the tunnel or outside the sphere. Let $F_R$ be the magnitude of the gravitational force on the apple when it is located at the planet's surface. How far from the surface is there a point where the magnitude is $\frac{1}{3} F_R$ if we move the apple (a) away from the planet and (b) into the tunnel?