An interesting experiment to perform involves a helium balloon tied to the seat of a car. As the car accelerates forward, the helium balloon will lean forward. There are two distinct ways of explaining why. The first involves the inertia of the air. An acceleration forward compresses the air in the rear of the car. The increased pressure in the rear of the car forces the balloon forward to the area of lower pressure. Just imagine the acceleration of the car as being equivalent to a gravitational field pointing backward. We can call the vector sum of the gravitational fields the “local field.” Both approaches—pressure differences and local fields—can be used to explain the motion of the helium balloon. This chapter focuses on finding some local fields on an idealized spherical Earth.