Motion and Its Causes

Force and Newton’s Laws of Motion

How did Einstein describe the gravitational field?

In 2006 the International Astronomical Union defined the term “planet” for the first time. Pluto and seven other objects are now called “dwarf planets.” One of these five, Eris, is 27% more massive than Pluto.

Einstein in his General Theory of Relativity showed that the gravitational field was actually a distortion of space-time caused by the mass of the object. Because space-time has four dimensions that are very difficult to visualize, the distortion is best seen with a two-dimensional model. Often the model consists of a rubber sheet in which a bowling ball is placed (see illustration on page 39). The sheet is pulled down by the ball, which represents the sun. Earth is a tiny ball that is placed on the sheet and given a push perpendicular to the direction of the sun. This ball “orbits” the sun a few times until friction causes it to speed up and spiral into the sun. In the words of physicist John Wheeler:


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