Potenta

Meet Potenta, a book on a high shelf who is storing energy just by being up there.

Lifting an object against gravity takes work, and that work is stored as gravitational potential energy. Near Earth's surface, where the gravitational field strength g is nearly constant, the stored energy is E_p = mgh, where h is the height above a chosen reference level. Let the object fall and the stored energy turns into kinetic energy.

A 2 kg book on a shelf 1.5 m above the floor has E_p = 2 × 9.81 × 1.5 ≈ 29.4 J relative to the floor. If it falls, ignoring air resistance, it reaches the floor with about 29.4 J of kinetic energy.

Hydroelectric dams turn the potential energy of water held high into electricity, and pumped-storage plants pump water uphill when power is cheap to store energy for later. Roller coasters trade height for speed again and again.

Only changes in potential energy matter; the zero level is a free choice. The formula mgh also only holds close to the surface; for satellites and planets, the full form from Newton's law of gravitation is needed.