1. Gravitation Inside Earth
A uniform shell of matter exerts no net gravitational force on a particle located inside it.
This means that the sum of the force vectors on the particle from all the elements is zero.
Uniform Earth
Real (nonuniform) Earth
The force on the particle actually increases as the particle begins to descend. The force reaches a maximum at a certain depth and then decreases as the particle descends farther.

2. Gravitational Potential Energy
In Section 8-4, we discussed the gravitational potential energy of a particle–Earth system. We were careful to keep the particle near Earth’s surface, so that we could regard the gravitational force as constant. We then chose some reference configuration of the system as having a gravitational potential energy of zero. Often, in this configuration the particle was on Earth’s surface.