One of the practice AP questions in this unit's packet gave me a little trouble when it asked what the force of gravity would be at the center of a planet. Mathematically, it would make sense that gravity is strongest at the center of the planet because gravitational force is inversely proportional to the square of the distance between the centers of mass of the two objects. But if gravity is a product of mass, then once the object reaches the center, it would be equally pulled in all directions by the two halves of the planet. Would this mean that one would be weightless at the center of the Earth? According to something called the Shell Theorem, no gravitational force is exerted on an object inside a symmetrical spherical shell regardless of the size of the shell or the objects location within it. If this is true, then that would allow a person to scoop out a large perfectly spherical section in the center of the earth and enjoy weightlessness. Of course the pressure at the center of the Earth would likely crush such a structure, but if a material that could withstand such heat and pressure could be synthesized, then one could theoretically float around deep beneath the surface. Of course any of this is incredibly impractical, but it does give scientific credence to the Dwayne "The Rock" Johnson movie, Journey to the Center of the Earth (not that such a masterpiece needed it).