The Law of Conservation of Energy
Seth Malone | September 17, 2026
The total energy inside an isolated system must remain constant.
This means that energy can neither be created nor destroyed. Energy can be transferred from one closed system to another, converted from potential to kinetic energy, or even converted into matter (thanks to Einstein’s famous E = mc2 equation). But the total amount of energy present in the system remains the same.
You may think you can create your own energy by moving, but you only have that energy from the food you ate—which, ultimately, mostly came from a plant photosynthesizing sunlight. The sun gets its energy from fusing hydrogen into helium, which eventually runs out (after all, the sun can’t just create energy).
If you drop a ball in a free fall, it won’t bounce back to its starting spot—no matter how bouncy the ball is. When it hits the ground, some of the ball’s kinetic energy is transferred into the ground and the surrounding air as heat and sound. Because it now has less mechanical energy, the ball will come short of its starting position. (Of course, if you throw it and dump extra energy into it yourself, it could bounce much higher, depending on how hard you throw it.)
This happens on the atomic level too. When a high-energy photon of light hits an atom, it can excite an electron into a higher energy state. This excited state, however, is temporary. When the electron falls back into a lower-energy state, the energy it gained has to go somewhere—and it often emits a photon of light back out. This is how many glow-in-the-dark objects work, and it’s also connected to how astronomers can detect what different stars are made of.