A fluid is a substance in which the constituent molecules are free to move relative to each other conversely in a solid the relative positions of molecules remain essentially fixed under non destructive conditions of temperature and pressure. Fluid mechanics is a subject with almost endless ramifications and the account that follows is necessarily incomplete some knowledge of the basic properties of fluids will be needed a survey of the most relevant properties is given in the next section for further details see thermodynamics and liquid. Basic fluid mechanics laws dictate that mass is conserved within a control volume for constant density fluids thus the total mass entering the control volume must equal the total mass exiting the control volume plus the mass accumulating within the control volume mass in mass out mass accumulating m in mout m acc 34