Animate
A block is suspended from a fixed support by a rubber band. When held in place by the green stick, the rubber band is completely relaxed. When the green stick is pulled away, the block oscillates vertically under the action of gravity and a Hooke's Law type spring force. The red line (marked y = 0) is the position at which both gravitational and elastic potential energy are taken to be 0. The system includes the block, Earth, and band. No external forces act on the system. (Scroll down.)
When you play the animation, 4 sets of blue, red, and green bars labeled A-D will appear. One of these sets represents the kinetic (blue), elastic potential (red), and gravitational potential (green) energies. Which set is the correct one? (Note the indicator of positive, 0, and negative energy shown to middle left.)