What is a Weight Process in Thermodynamics?

[Saladsamurai]

Here we go again! I am taking a graduate thermodynamics course and I feel like I am in physics 1 all over again. The definitions that I took for granted early on in my education I am really wanting to make sure that I have a firm understanding of before I progress. I am reading from the thermodynamics text by Gyftopoulos and Baretta. I am reading the energy chapter and he is in the midst of defining a weight process. I think that I understand it for the most part, but I am getting hung up on some of the details (the devil is in the details!).

Here are the two paragraphs that I am currently reading:

I believe he is just intuitively proving the Work Energy theorem, but I am getting a little lost. Why is the force F constant? I see that we have attached the pulley to the point mass in its final state where it has velocity, but are they calling that V1 or v2? I believe that they are calling it V1. That is, by using the term annulled, it appears that they are working backwards in a sense. They are using the weight to bring the point mass from its final state back to its initial.

I am also confused by the whole ||W| - Mg| / Mg << 1 thing. What is that all about now?

Sorry if these seem stupid, but I really need to talk this out with someone. I know from other students that this entire book takes a really different approach to thermodynamics, so I really want to stay on top of the definitions used early on.

Thanks!
Casey

 

[LightHero]

That's a great question, Casey! What the author is doing in this section is proving the Work-Energy theorem, which states that the work done on an object is equal to the change in its kinetic energy. This theorem is derived from the basic equation of motion, F = m · a, which states that the force applied to an object is equal to its mass multiplied by its acceleration. The force F is considered constant because the pulley is attached to the point mass in its final state, where it has velocity v1. This allows us to use the equation W = F · ds, where W is the work done and ds is the displacement of the point mass, to calculate the work done on the point mass. The expression ||W| - Mg| / Mg << 1 is used to show that the work done in a weight process is equal to the change in kinetic energy of the point mass. In other words, if the work done is much less than the potential energy, then the change in kinetic energy is equal to the work done. I hope this helps clarify things for you!