1-1 INTRODUCT00N
Mechanics is the oldest physical science that deals with both stationary andmoving bodies under the influence of forces．The branch of mechanics thatdeals with bodies at rest is called statics,while the branch that deals withbodies in motion is called dynamics．The subcategory fluid mechanics isdefined as the science that deals with the behavior of fluids at rest(fluid sta．tics)or in motion(fluid dynamics),and the interaction of fluids with solids0r other fluids at the boundaries．Fluid mechanics is also referred to as fluiddynamics by considering fluids at rest as a special case of motion wlth zerovelocity(Fig．1一l)．
Fluid mechanics itself is also divided into several categories．Thc study ofthe motion of fluids that can be approximated as incompressible(such as liq．uids．especially water,and gases at low speeds)is usually referred to as hydro．dynamics．A subcategory of hydrodynamics is hydraulics,whch deals withliquid flows in pipes and open channels．Gas dynamics deals with the flow offluids that undergo significant density changes,such as the flow of gasesthrough nozzles at high speeds.The category aerodynamics de:als with theflow of gases(especially air)over bodies such as aircraft,rockets,and automo．biles at high or low speeds.Some other specialized categories such as meteo rology,oceanography,and hydrology deal with naturally occurring flows．What!s a Fluid?
You will recall from physics that a substance exists in three primary phases:solid．1iquid,and gas．(At very high temperatures,it also exists as plasma．)A substance in the liquid or gas phase is referred to as a fluid．Distinctionbetween a solid and a fluid is made on the basis of the substance's ability toresist an applied shear(or tangential)stress that tends to change lts shape．Asolid can resist an applied shear s~ess by deforming．whereas a flMzddcfoFillS continuously under the influence of a shear stress,no matter howsmall．In solids．stress is proportional to strain,but in fluids,stress is pro．portional to strain rate．When a constant sheqr force is applied,a solid even．tually stops deforming at some fixed strain泗gIC,whereas a fluid neverstops deforming and approaches a constant rate of strain．
Consider a rectangular rubber block tightly placed between two plates．Asthe upper plate is pulled with a force F while the lower plate is held fixed,the rubber block deforms,as shown in Fig．1-2．The angle of deformation 0／(called the shear strain or angular displacement)increases in proportion tothe applied force F．Assuming there is no slip between the rubber and theplates．the upper surface of the rubber is displaced by an amount equal tothe displacement of the upper plate while the lower surface remams station．ary．In equilibrium,the net force acting on the upper plate In the horizontaldirection must be zero,and thus a force equal and opposite to F must beacting on the plate．This opposing force that develops at the plate-rubberinterface due to friction is expressed as F'rA,where丁is the shear stressand A is the contact area between the upper plate and the rubber．When theforce is removed,the rubber returns to its original position．This phenome．non would also be observed with other solids such as a steel block providedthat the applied force does not exceed the elastic range．If this expenmentwere repeated with a fluid(with two large parallel plates placed in a largebody of water,for example),the fluid layer in contact with the upper platewould move with the plate continuously at the velocity of the plate no mat—ter how small the force F．ThC fluid velocity would decrease with depthbecause of friction between fluid layers．reaching zero at the lower plate．
You will recall from statics that stress is defined as force per unit areaand is determined by dividing the force by the area upon which it acts．Thenormal component of a force acting on a surface per unit area is called thenormal stress,and the tangential component of a force acting on a surfaceper unit area is called shear stress(Fig．1—3)．In a fluid at rest．the normalstress is called pressure．A fluid at rest is at a state of zero shear stress．Wbcn the walls are removed or a liquid container is tilted．a shear developsas the fiquid moves to re—establish a horizontal free surface．
In a fiquid,groups of molecules can move relative to each other,but thevolume remains relatively constant because of the strong cohesive forcesbetween the molecules．As a result．a liquid takes the shape of the container itis in,and it forms a free surface in a 1arger container in a gravitationaleld．Agas,On the other hand,expands until it encounters the walls of the contmnerand fills the entire available space．This is because the gas molecules arewidely spaced,and the cohesive forces between them are very small．Unlikeliquids,a gas in an open container cannot form a free surface(Fig．1—4)．
Although solids and fluids are easily distinguished in most cases,this dis一血cfion is not so clear in some borderline cases．For example．asphalt appearsand behaves as a solid since it resists shear stress for short periods of time．When these forces are exerted over extended periods of time,however,theasphalt deforms slowly,behaving as a fluid．Some plastics．1ead,and slurrymixtures exhibit similar behavior．Such borderline cases are beyond the scope0f ois text．The fluids we de址wi出in this text will be clearly recognizable asfluids．
Intermolecular bonds are strongest in solids and weakest in gases．Onereason is that molecules in solids are closely packed together,whereas ingases they are separated by relatively large distances(Fig．1—5、．The mole．cules in a solid are arranged in a paUem that is repeated throughout．Becauseof the small distances between molecules in a solid．the attractive forces ofmolecules on each other are large and keep the molecules at fixed positions.
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