788 Work and Energy New words Conservative and non- conservative forces(保守力与 非保守力) Potential energy (势能) Conservation of mechanical energy (机械能守恒) Conservation of energy(能量守恒)7&8 Work and Energy New words Conservative and non-conservative forces（保守力与 非保守力） Potential energy（势能） Conservation of mechanical energy （机械能守恒） Conservation of energy（能量守恒）