I'm currently learning about different types of compressional work. The book I'm using covers mostly just isothermal and adiabatic processes, which make sense. Isothermal being so slow that everything equilibriates while adiabatic is so fast that heat cannot escape. However, the book briefly mentions that we can only say F=PA if the process is quasistatic, i.e. the gas is compressed faster than the relaxation time (speed > speed of sound). Why would F=PA still not apply in the case of a "non-quasistatic" process? Wouldn't it just be a differential process ? I.e. there'd be a pressure gradient of some sort. Or is there something I'm missing? And how can a process be adiabatic but also quasistatic? Moreover, what happens when something is compressed at speeds faster than the speed of sound? How does the medium behave?