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154 SCIENCE12.6 Fluid FrictionYou know that air is very light and thin.Yet it exerts frictional force on objectsmoving through it. Similarly, water andother liquids exert force of friction whenobjects move through them. In science,the common name of gases and liquidsis fluids. So we can say that fluids exertforce of friction on objects in motionthrough them.The frictional force exerted by fluidsis also called drag.The frictional force on an object in afluid depends on its speed with respectto the fluid. The frictional force alsodepends on the shape of the object andthe nature of the fluid.It is obvious that when objects movethrough fluids, they have to overcomefriction acting on them. In this processthey lose energy. Efforts are, therefore,made to minimize friction. So, objectsare given special shapes. Where do youthink the scientists get hints for thesespecial shapes? From nature, of course.Birds and fishes have to move about influids all the time. Their bodies musthave evolved to shapes which wouldmake them not to lose much energy inovercoming friction. You read aboutthese shapes in Class VI. Look carefullyat the shape of an aeroplanes (Fig.12.17). Do you find any similarity in itsshape and that of a bird? In fact, allvehicles are designed to have shapeswhich reduce fluid friction.Fig. 12.17 : Similarity in shapes of an aeroplane and a birdFig. 12.16 : Ball bearings reduce friction%u00a9 NCERTnot to be republished