Well I'm not an engineer but I think hydraulics deals with compression of an incompressible fluid. When the pistons activate there are a number of forces in action. The main one would be the kinetic coefficient of the contained atmosphere and the liquid. Obviously these two are dynamic.
-Edit: didn't read 95sleeper's response. As he stated, however do note that both are dynamic in terms of force application. (I love these thoughts, gives great MCAT practice!
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