Energy sits in stores, kinetic, gravitational, chemical, and shifts between them when something happens. Through every swing, fall and collision, the total never changes. That single rule, the conservation of energy, runs through all of physics.
Energy is held in stores (kinetic, gravitational potential, elastic, chemical, nuclear, internal/thermal, electrostatic) and moved between them by transfers (mechanical work, electrical, heating, and waves). Energy cannot be created or destroyed, only transferred; the total stays constant.
Watch a swinging pendulum trade its energy between stores. At the top of a swing it is all gravitational store and momentarily still; at the bottom it moves fastest and the store is all kinetic. Drag the bob to set its starting height, then switch on air resistance and watch what really happens: the swing dies down, but the energy is not lost. It moves into the thermal store of the surroundings, and the total never changes.
| Energy stores | Ways energy is transferred |
|---|---|
| kinetic (movement) | mechanically, by a force doing work |
| gravitational potential | electrically, by a current |
| elastic (strain), chemical, nuclear | by heating |
| internal (thermal), electrostatic | by waves (light, sound, radiation) |
When energy seems to be "lost", it has actually been transferred to a less useful store, usually the thermal store of the surroundings, by friction or air resistance. The total energy is always conserved. Saying energy is created, destroyed or used up loses marks; say it is transferred from one store to another.