I was thinking about mechanics and energy and stumped myself with this scenario I made up:
Suppose you are in a room and you are motionless. In the same room, a 1 Kg ball is moving to your right with a speed of 1 m/s. You arrest the ball's motion, so the ball loses its one joule of kinetic energy and also becomes motionless.
However, I lied. The room is actually in space, and is going to my left at a speed of 100 m/s, and the ball is also going to the left from my perspective, but slightly slower at 99 m/s. From my perspective, the ball gains kinetic energy, despite the opposite being true from your perspective.
How is this possible?