We have to convert physical equation into laplace equation in order to make a block diagram. Some basic about mathematic, especially differential equation must be considered. First look at the electrical system with Resistor, Inductor and capacitor element (RLC).
According to kirchhoff law we have:
If we want to drop of voltage at capacitor as an output, we get a blog diagram. Inside the block diagram is a transfer function, G. The transfer function is a comparison between output and input. Instead of using equation in time domain, we use s-domain (laplace).
This is a 2-orde system. Other system can be build similar to electrical, for example, mechanical system. We have three component dumper, spring, and mass. That element behave similar to those in electrical system (RLC).
We use Newton law to get mathematical model to this system. The force equal against force at mass, spring, and dumper.
If we want displacement of mass as an output, we will get a transfer function. Remember the force at mass have connection with displacement of spring (kx).