[automagp68]

Can someone tell me how this is the case

We all know the energy balance for a closed system is the following

(Qin-Qout)+(Win-Qout) = ΔU + ΔPE + ΔKE

But how do you get U = Q - W out of that?

I don't understand how you get from one equation to the other and when its appropriate to use one or the other.