My professor (for some weird reasons) makes us write "system [body(ies)]" whenever we are asked to solve a physics problem for work and energy. Systems make me completely confused because sometimes if we have two or more systems, then the net work becomes zero. My question is this: How do I know when the net work becomes zero? Please give me examples of writing "system _" when solving a question. If the above doesn't make sense, here is an example: Suppose there is a mass on the air with 0 initial velocity. Find the final velocity (assuming force of gravity is the only force acting on it). The way our professor asked us to solve it is by writing "system mass & earth"... in which case the work due to gravity apparently becomes zero (??). Please explain situations where net work becomes zero and where net work is not zero when I include earth on my system... Sorry for the confusion!!