First of all, sorry for my bad english. Although it's a fluidic system, I put this topic here because it has an electrical equivalent (ON-TOPIC). Consider the following system. Using Matlab (simulink), observe the unitary step response to the interval [0, 5] seconds. Obtain the graph of the height (h1 and h2) of water in both tanks. Initial conditions are zero. qi, qo = water flow (inbound and outbound) R1, R2 = fluidic resistance A1, A2 = area of the tanks I have some doubts about one thing in this system but first, my resolution. The areas of the tanks are constants. General conditions: Now, considering q1 as the water flow through R1: Then, using Laplace transform: Simplifying, Now, I can get the following block diagram. (considering water density=1) colours: Yellow - Qi(s) Violet - H1(s) Blue - H2(s) Red - Qo(s) Well, my doubt is a bit silly. I don't know where or when I can use H=1... One thing that I know is when h2(t)<1, qo(t) should be 0 (right?) but in my diagram this isn't true. (h2(5)=0.20 and qo(5)=1 approximately) well, any help? how to solve this?