Hello all! I am reading a thermodynamics text for a graduate course and I am running into some confusion early on. All throughout physics I thought that I knew what a system was; but, apparently I do not . Here is part of their definition and an example that I find confusing. It concerns the relationship between what is defined as a system and the external forces on that system. Here is their example that confuses me: Any thoughts on this? I used to think that I could define a system in any manner that suits my needs for analysis; but, it now seems that is not the case. For anyone tha might have the text, I am reading from Thermodynamics: Foundations and Applications by Gyftopoulos and Baretta. Edit Maybe I should provide an example of what I believe is a system and someone can point out wherein lies the difference. An example that I feel is analogous to their 'magnet' example is the following. Suppose we have two blocks attached to one another by a string, one above the other (i.e. one block is dangling from the other via the string). There is a force built up in the string that imparts its effects to each block. I would say that I could isolate one block by making an imaginary 'cut' in the string to expose the force being exerted on each of the blocks. I could draw a free body diagram of the each block by simply drawing the block and the tensile force on each block as well as the weights of the blocks. Can I not now call each block a system?