EN: The law of conservation of energy states that energy cannot be created or destroyed. - Total energy stays constant in an isolated system. - What changes is the form of energy and where it is (system vs. surroundings). - Some transformed energy becomes less useful (often as thermal energy and sound) due to friction and other processes.

中文：能量守恒定律：能量不会凭空产生，也不会凭空消失。 - 对于孤立系统，总能量保持不变。 - 改变的是能量的形式以及能量所在的位置（系统或环境）。 - 由于摩擦等原因，部分能量会转化为较难利用的热能/声能，但总量仍守恒。