I think I have generally figured out when it is appropriate to use the relativistic expression for energy (
) versus the classical (
), but I want to make sure and/or learn of any other ways ETS might want us to use one or the other. I've found in general that if a question is put in terms of, for instance, MeV/c^2 it is logical to use the relativistic case, and also I've found that often the use of the word "free" (i.e. a free electron) can also suggest to use relativistic. Are there any other general times to use one or the other?