Conservation of energy
Energy cannot be created or destroyed, only converted between forms — the total energy of an isolated system stays constant.
Practise this conceptEnergy is never created or destroyed — it only ever changes form or moves from one object to another. A pendulum swinging back and forth trades potential energy for kinetic energy and back again on every swing, and if nothing else were involved, it would keep swinging at the same height forever.
Nothing else is never quite true, though. Air resistance and friction at the pivot slowly convert some of that mechanical energy into heat, which is why real pendulums eventually stop. The energy hasn't vanished — it has spread out into the surrounding air and the pivot as a small, hard-to-recapture rise in temperature.
This is the pattern behind every "energy seems to disappear" puzzle in mechanics: a ball that stops rolling, a spring that stops bouncing, a car that brakes to a halt. Track the heat and sound generated along the way, and the total is always exactly what you started with.
Key ideas
Requires: Work and energy
Unlocks: Kinetic theory of gases, The laws of thermodynamics, Newtonian gravitation
Formulas
| Symbol | Name | Unit |
|---|---|---|
| velocity | m/s (metre per second) | |
| distance | m (metre) |
Common misconceptions
- When a ball rolls to a stop due to friction, its energy simply disappears.
- Kinetic energy is always conserved during a collision.
- The total energy of a system decreases whenever energy changes from one form to another.