Hi, After reading several resources about work and energy, I am confused about the conditions that should be satisfied in order to be able to apply work energy theorem. It seems that textbooks have different arguments about this issue. I can summarize what textbooks say in three different opinions about the condition to apply work energy theorem: 1) Work energy theorem is only applied to particles. 2) Work energy theorem should include both internal and external works. 3) The change in the system should be restricted to only kinetic energy. Which is true? Which is accepted by the physics community? Thanks in advance.