The way you move through space is a consequence of the shape of spacetime. General relativity is the modern understanding of gravity, where astronomers think of gravity not as a force that pulls you, but rather as a curvature of space and time. Note that if you roll two ping pong balls at the same time, the inner one goes around faster than the outer one, just as Kepler's Laws of Planetary Motion predict. Depending on how you roll the ping pong ball, you can get circular or elliptical orbits (like planets, moons and asteroids have), or highly elongated parabolic or hyperbolic orbits that do not close (like many comets have). (3) CURVED SPACE ORBITS: With practice, you can make your ping pong balls "orbit" the mass in the center of the table. Now if you roll a ping pong ball across the table, it does NOT travel in a straight line! Instead, its path is curved, bending toward the mass in the center of the table! Place your heavy mass or rock in the middle of the table. This demonstrates motion in the presence of masses ("under the influence of gravitational forces").
If you roll a ping pong ball straight across the table it will go across without changing direction (until it falls off the edge of the Universe on the far side). A particle that is acted on by no forces travels in a straight line.
This demonstrates motion in the absence of masses ("in the absence of gravitational forces"). There are two modes of demonstration with the table.
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