The Origin of Tidal Forces

This animation illustrates the origin of tidal forces. Imagine three identical billiard balls placed some distance from a planet and released. The closer a ball is to the planet, the more gravitational force the planet exerts on it. Thus, a short time after the balls are released, the yellow 1-ball has moved a short distance, the green 2-ball has moved a longer distance, and the red 3-ball has moved a still longer distance. From the perspective of the center ball (the 2-ball), a force seems to have pushed the 1-ball away from the planet, and a force seems to have pulled the 3-ball toward the planet. These forces are called tidal forces.