Gravity Assist on Skates
The gravity assist a spacecraft gets from a nearby planet is similar to a move you might see in a roller derby slingshot maneuver. Imagine a skater gliding steadily along. A second skater approaches, grabs skater #1’s hand for a moment, swings past, and lets go. This brief encounter causes skater #2 to change direction and pick up speed, and skater #1 to lose a little speed.