Hi, Our lecturer explained us the Reynold Transport theorem, its derivation , but I don't get where the - sign in control surface 1 comes from? He said that the Area goes in opposite direction compared with this system. I can't visualise this on our picture. Can you please help me understand why we have the negative sign on the control surface 1 and at the end of the theorem we have +ve everywhere? The pictures are attached below Fig1-Illustrates the - sign enquiry with regards to control surface 1. Why is it - here? and not + Fig2-Illustrates the final form of the Reynolds Transport theorem where all signs are +, why? How to know when the Area is in same direction as the system velocity ? and how to know when the Area goes opposite direction with regards to the system velocity? Thank you in advance.