CHAPTER II. 



MOTION OF WATER IN PIPES AND OPEN CHANNELS. 



102. Resistance of Friction. When a thin plate 

 with sharp edges, completely immersed in water, is moving 

 edgeways through the water, a certain resistance is expe- 

 rienced, which must be overcome by an external force. 

 This resistance acts along tangentially between the plate 

 and the water, and so far is analogous to the friction be- 

 tween solid surfaces, but it follows quite different laws, 

 which have been obtained from many observations and ex- 

 periments, and which may be stated as follows:* 



(1) The resistance of friction is entirely independent of 

 the pressure on the surface. 



(2) It varies as the area of the surface in contact with 

 the water. 



(3) It varies nearly as the square of the velocity, f 



Hence, if R be the resistance of friction, 8 the area of the 

 surface, and v the velocity, these laws may be expressed by 

 the formula, 



(1) 



where /is called the "coefficient of friction," as in the fric- 

 tion of solid surface?. The value of / depends on the 

 smoothness of the surface; thus, for thin boards, with a 

 clean, varnished surface, moving through water, it is found 



* Cotterill's App. Mechs., p. 468. 



t At low velocities, of not more than 1 inch per second for water, the resistance 

 varies nearly as the first power of the velocity. At velocities of } foot per second, 

 and greater velocities, the resistance varies more nearly as the square of tbe ve- 

 locity. 



