Science Puzzle
Where Did the Energy Go?
Pull a pendulum to one side and let go. It swings back and forth, but watch closely: every swing ends slightly lower than the one before. After a few minutes it hangs completely still.
You were taught that energy can never be created or destroyed. The pendulum clearly had energy when you lifted it. So where did it all go?
The Answer
It leaked away as heat, one tiny bit per swing. Lifting the bob gave it stored gravitational energy. On the way down that became movement energy, then back into height on the far side, over and over.
But each pass, the bob shoves air molecules out of its way, and the string rubs at the pivot. Every shove and every rub converts a little of the swing into heat, warming the air and the pivot by an amount far too small to feel. The energy was never destroyed. It was spread out into the room, in a form too disordered to gather up and reuse.
This is why no machine, however clever, can run forever without a power source. Every real machine leaks energy as heat somewhere.
The principle: Conservation of energy. Energy is never created or destroyed, only converted between forms. Friction and air resistance convert useful movement into heat spread thinly through the surroundings.