Earth's moon is held in orbit by an attractive gravitational force between Earth and the moon. Tides on Earth are due mainly to the gravitational pull of the moon on Earth. Any two masses whether or not Earth and the moon, experience a mutual gravitational force that tends to pull them together. A mass in a position to be pulled to another position by a gravitational force has gravitational potential energy. Anything—water, a book, a parachutist, a molecule in the atmosphere, etc.—has gravitational energy if it is in a position to move closer to the center of Earth. Ordinarily, something has to do work to get the object to the elevated position. A book on the floor of a room has no gravitational energy if it cannot move lower than the floor. But if you lift the book, and place it on top of...