RULES OF THUME FOR MECHANICAL ENGINEERS
One must always use a consistent set of units. Primary units are mass, length, time, and temperature. A unit system is called consistent when unit force causes a unit mass to achieve unit acceleration. In the U.S. system, this system is represented by the (pound) force, the (slug) mass, the (foot) length, and the (second) time. The slug mass is defined as the mass that accelerates to one ft/& when subjected to one pound force (lbf). Newton’s second law, F = ma, establishes this consistency between force and mass units. If the mass is ever referred to as being in lbm (inconsistent system), one must first convert it to slugs by dividing it by 32.174 before using it in any consistent equation.
Because of the confusion between weight (lbf) and mass (lbm) units in the U.S. inconsistent system, there is also a similar confusion between density and specific weight units. It is, therefore, always better to resort to a consistent system for engineering calculations.
Fluid statics is the branch of fluid mechanics that deals with cases in which there is no relative motion between fluid elements. In other words, the fluid may either be in rest or at constant velocity, but certainly not accelerating. Since there is no relative motion between fluid layers, there are no shear stresses in the fluid under static equilibrium. Hence, all bodies in fluid statics have only normal forces on their surfaces.
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