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on the translational motion of the ball! For the same reason, a baseball’s cover helps A number of devices operate by means of the pressure differentials that result from differences in a fluid’s speed. For example, a stream of air passing over one end of an Summary 437 Figure 14.23 Because of the deflection of air, a spinning golf ball experiences a lifting force that allows it to travel much farther than it would if it were not spinning. Figure 14.24 A stream of air pass- ing over a tube dipped into a liquid causes the liquid to rise in the tube. The pressure P in a fluid is the force per unit area exerted by the fluid on a surface: (14.1) In the SI system, pressure has units of newtons per square meter (N/m 2 ), and 1 N/m 2 ! 1 pascal (Pa). The pressure in a fluid at rest varies with depth h in the fluid according to the expression (14.4) where P 0 is the pressure at h ! 0 and $ is the density of the fluid, assumed uniform. Pascal’s law states that when pressure is applied to an enclosed fluid, the pressure is transmitted undiminished to every point in the fluid and to every point on the walls When an object is partially or fully submerged in a fluid, the fluid exerts on the object an upward force called the buoyant force. According to Archimedes’s principle, the magnitude of the buoyant force is equal to the weight of the fluid displaced by the object: (14.5) You can understand various aspects of a fluid’s dynamics by assuming that the fluid is nonviscous and incompressible, and that the fluid’s motion is a steady flow with no Two important concepts regarding ideal fluid flow through a pipe of nonuniform size are as follows: 1. The flow rate (volume flux) through the pipe is constant; this is equivalent to stat- ing that the product of the cross-sectional area A and the speed v at any point is a equation of continuity for fluids: (14.7) A 1 v 1 ! A 2 v 2 ! constant B ! $
fluid
gV P ! P 0 % $ gh P
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