…build and operate a mechanical device to simulate, remarkably well, what happens when a robotic spacecraft takes advantage of a “gravity assist” on its interplanetary trajectory through the solar system. The device substitutes a strong, moving magnet for a massive orbiting planet, and a steel bearing ball (BB) for a spacecraft. The “magnetic assist” it provides to the BB is closely analogous to the “gravity assist” a spacecraft can receive from a planet moving in its solar orbit. Link.
Build a Gravity Assist