Hi I have a few questions regarding gas dynamics.... 1. when M=1, dA=0....what does this mean? Does it imply that if we have a converging (subsonic) nozzle with its exit mach no,say 0.8, and then direct the flow to a constant area tube, then we have to attain M=1? 2. When we know the length of a duct, and the entering mach no, can we predict from only this information whether there will be a shock or not? (i.e what is the minimum info we require to predict whether there will be a shock or not?). 3. Is it true that while a subsonic flow can be continuously accelerated to supersonic flow, the converse (i.e continuous deceleration of a supersonic flow to subsonic speed) is not true and has to take place via a shock wave?(but I thought this was possible in a diverging diffusor..... Thanks a lot.