Please help me qualitatively in the following points : 1) If in a system(consider a cylinder) fixed with a piston , if the piston is moved suddenly then how can a process be adiabatic. 2) I understood that the process would be irreversible but, if the process is adiabatic then is the relation PV^y = constant (where y is gamma, P is pressure, V is volume) is true for irreversible process too. (In many books they have written that the relation is true only for reversible processes) In the following question , I am not getting the real essence of the mechanics of the process. Please explain: Question: A gas is enclosed in a cylindrical can fitted with a piston. The walls of the can are adiabatic. The initial pressure, volume and temperature of the gas are 100 kPa, 400 cc (cubic cm) and 300 K respectively. The ratio of the specific heat capacities of the gas is Cp/Cv=1.5 . Find the pressure and temperature of the gas if it is (a)suddenly compressed to 100 cc (cubic cm). (b)slowly compressed to 100 cc (cubic cm). Here, the answer to both the cases is given same by taking PV^y = constant (where y is gamma). How would it be??