neither created nor destroyed by any process. It is often termed the 

 continuity condition. The second principle is a form of Newton's 

 second law which states that the acceleration a body (in this case, an 

 infinites imally small element of fluid) experiences is proportional 

 to the net sum of all forces acting on the body and is in the direction 

 of the net force. Implicit in this statement is the principle that once 

 a body is in motion, it can only be brought to rest by imposing a force 

 in the direction opposite to the direction of motion in order to 

 "decelerate" the body. Consequently, a volume of fluid set in motion 

 will tend to remain in motion even after the force causing the flow is 

 no longer acting. The opposing force which normally acts to decelerate 

 a fluid is friction. Decelerating a fluid may take days or weeks after 

 the driving forces have ceased. 



Conservation of energy is a consequence following directly from these 

 principles and the relationship between density, pressure, and temperature. 



The principles can be expressed mathematically by (a) a continuity 

 equation, (b) equations of motion termed the Navier-Stokes equations, 

 and (c) an equation of state. All knowledge of lake circulations, 

 currents, and waves follows directly from these equations and the 

 physical laws they represent. 



It is important to recognize the scale of a particular phenomenon 

 under study since it will determine which aspects of the flow will be 

 considered the mean flow and which aspects will be considered turbulence. 

 Generally, it is easier to understand the aspects of a particular physical 

 process when only one scale of motion is considered at a time, e.g., in 

 many problems, the largest scale under investigation represents current 

 motion while smaller-scale motions, such as waves, appear as irregularities 

 in the mean flow. The small-scale phenomena are usually identified as 

 turbulence when they appear to be disorganized and to be constantly 

 changing their appearance. It is not uncommon to find that a particular 

 flow phenomenon is regarded as the mean flow at one scale and turbulence 

 at another scale. The definition depends on the flow scale of primary 

 interest . 



As indicated, fluid motions once established continue for some time 

 after their generating mechanism has ceased to operate. An analogy 

 between the water motion in a lake, e.g. , Lake Michigan, and the motion 

 of a wheel is apropos : stroking the top rim of a wheel toward the right 

 will produce a downward motion on the right, a motion toward the left at 

 the bottom and an upward motion to the left of the wheel. The motion of 

 the entire system may continue for some time after one stops stroking 

 the wheel. If the fluid in one part of Lake Michigan is forced to move 

 from west to east, a return flow from east to west must occur in some 

 other part of the basin. This may be associated with flow in a vertical 

 plane or may involve flow in a horizontal plane. The fluid motion will 

 continue after the generation force has ceased (as a wheel will continue 

 to spin for some time after the addition of energy ceased) until the 



