能量守恒定律（英语：law of conservation of energy）阐明，孤立系统的总能量 保持不变。如果一个系统处于孤立环境，即不能有任何能量或质量从该系统输入或输出。能量不能无故生成，也不能无故摧毁，但它能够改变形式，例如，在炸弹爆炸的过程中，化学能可以转化为动能。

从能量守恒定律可以推导出第一类永动机永远无法实现。没有任何孤立系统能够持续对外提供能量。

In physics, the law of conservation of energy states that the total energy of an isolated system remains constant—it is said to be conserved over time. Energy can neither be created nor destroyed; rather, it transforms from one form to another. For instance, chemical energy can be converted to kinetic energy in the explosion of a stick of dynamite.