Why does F=PA fail when gas is compressed faster than the relaxation time?

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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?
 
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randomafk said:
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.
Adiabatic processes can be slow. Adiabatic simply means no heat flow occurs between the system and surroundings. Slow adiabatic processes can occur, for example, if the system is thermally isolated.

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?
Quasi-static does not mean that the gas is compressed faster than the relaxation time. "Quasi-static" means that the process occurs at conditions arbitrarily close to equilibrium.

And how can a process be adiabatic but also quasistatic?
The process has to occur slowly and without heat flow. In the real world (eg in an engine) this is hard to do. One would need very good insulation.

Moreover, what happens when something is compressed at speeds faster than the speed of sound? How does the medium behave?
If something is compressed faster than the speed of sound you get a shock wave.

AM
 
randomafk said:
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 ?

E.g. second viscosity:
http://en.wikipedia.org/wiki/Bulk_viscosity