A shaft is drilled from the surface to the center of the earth (Fig. 12.25). As in Example 12.10 (Section 12.6), make the unrealistic assumption that the density of the earth is uniform. With this approximation, the gravitational force on an object with mass m, that is inside the earth at a distance r from the center, has magnitude F. = Gm/Emr/R; (as shown in Example 12.10) and points toward the center of the earth.
(a) Derive an expression for the gravitational potential energy U(r) of the object-earth system as a function of the object's distance from the center of the earth. Take the potential energy to be zero when the object is at the center of the earth.
(b) If an object is released in the shaft at the earth's surface, what speed will it have when it reaches the center of the earth?