So I have a doubt that's been bugging me for a very long time, and my teacher's response to it was vague. And I would be very grateful if someone would help me clear the misconceptions I have. Now, first of all, according to the first law of thermodynamics, [itex]\Delta[/itex]U = Q + W, where U is the internal energy of the closed system, Q is the heat transferred to the system, and W is the work done on the system. Here comes the part that I don't fully comprehend; what "kind" of work done are we referring to here? For instance, say we have a trolley at rest on a smooth surface; would doing work on the trolley (pushing it) increase it's internal energy? I mean, what am I missing out here? Is internal energy defined relative to a stationary system? So we don't consider work to be done "on" the system unless the system is actually "deformed"?