What fundamental property of energy is conserved?

• A. It can be created and destroyed
• B. It has no mass
• C. It cannot be measured
• D. It is conserved

Answer: (D)

Energy can be tracked as it changes form, but in an isolated system the total amount stays the same. That is the essence of energy conservation: energy may shift between kinetic, potential, thermal, chemical, and other forms, and may move between objects, yet the overall total remains constant. A simple example is a frictionless pendulum: as it swings, energy continually trades between height (potential) and speed (kinetic), and the sum of those energies stays constant. This is why the statement describing energy as conserved is the correct one. The other ideas conflict with this principle: energy isn’t something that can be created or destroyed in a closed system; while energy and mass are related through mass-energy equivalence, energy itself is a measurable quantity and can be observed through its effects.