A Roller Coaster illustrates the trade off between Potential and Kinetic Energy. Potential Energy is energy of position, specifically height. When the car is at the top of the coaster it has it's maximum Potential Energy. Kinetic Energy is energy of motion, specifically velocity. When the car is at the bottom of the coaster going through the dip, it has it's maximum Kinetic Energy. In between the top and bottom of the coaster, when the car is going up or coming down, is where Potential Energy and Kinetic Energy are under going a trade-off. Of course this is not a perfect trade off since some energy is lost due to friction and side-to-side motion of the car.