Stretchor

Meet Stretchor, a spring who stores energy every time she is stretched.

Stretching a spring takes work, because the spring pulls back harder the more you stretch it. That work is stored as elastic potential energy. For a spring that obeys Hooke's law, the force grows in proportion to the stretch, and the stored energy is half the spring constant times the stretch squared.

A spring with k = 500 N/m stretched by 0.10 m stores ½ × 500 × 0.10² = 2.5 J. Stretch it twice as far, to 0.20 m, and it stores four times as much, 10 J, because the stretch is squared.

The energy comes back when the spring is released, turning into kinetic energy. This only holds while the spring stays within its elastic limit. Stretch it too far and it deforms permanently and does not return all the energy.

Bows, catapults, trampolines, pogo sticks and wind-up toys all store energy this way. Robert Hooke published his law of springs in 1678, in Latin, as "as the extension, so the force".