Some examples in text book make me confused when these two works are discussed at the same time. One of the works is the (mechanical) work in work-energy theorem: [tex] \Delta K = \sum_iW_i, [/tex] where [tex]K[/tex] is the kinetic energy and [tex]W_i[/tex] was the work done by the [tex]i[/tex]-th force. The other is the (thermodynamical) work in the first law of thermodynamics: [tex] \Delta U = Q + W, [/tex] where [tex]U[/tex] is the internal energy of the system, [tex]Q[/tex] is the heat transfered, and [tex]W[/tex] is the work done on the system by surroundings. Are the two works the same when we want to use work-energy theorem and the first law of thermodynamics at the same time? Can any one give some criterion to distinguish these two works ? Thank you .