Physics
Energy Conservation
Quick fact
The total energy in the universe is constant; the energy from a single apple falling from a tree can be traced through countless transformations without ever being lost.
Why this is interesting
You've probably heard that energy can't be created or destroyed—but if that's true, why does a moving car eventually stop, and where does its energy go?
Read the full explanation
Understanding Energy Conservation
Imagine you have a ball sitting on a high shelf. It has stored energy called potential energy. When it falls, that stored energy changes into kinetic energy—the energy of motion. As the ball hits the ground, the kinetic energy transforms into sound, heat, and deformation of the floor. None of the original energy disappears; it just changes form. Energy conservation means that in any isolated system, the total amount of energy remains constant. If you add up all the different forms—kinetic, potential, thermal, chemical, electrical—the sum is always the same before and after an event.
A deeper explanation
The principle of energy conservation is a direct consequence of the time symmetry of physical laws: if the laws of physics do not change over time, energy is conserved (Noether's theorem). In practice, this means that energy can be transferred between objects (like a bat hitting a ball) or transformed (chemical energy in fuel becoming heat and motion), but the total quantity never changes. This is why perpetual motion machines are impossible—they would require creating energy from nothing. The first law of thermodynamics formalizes this: the change in internal energy of a system equals heat added minus work done. Energy conservation underpins everything from calculating how high a rocket can go to understanding why your phone battery gets warm when using it.