172 WORK AND PRESSURE OF A 8TKEAM OF WATER. 



(2) The pressure of a current upon a plane is equal to 

 the resistance suffered by the same plane when moving in 

 the same direction and with the same velocity through the 

 fluid ; therefore (3) will also represent the pressure which 

 the current, moving with the velocity v, would exert against 

 the plane at rest. Calling F the pressure, we have 



F = Icaw =?. (4) 



2# 



98. Work and Pressure of a Stream of Water. 



To find the work of a stream of water which impinges 

 perpendicularly upon the surface of a heavy body 

 which is itself in motion, and whose weight is very 

 great as compared with that of the impinging water. 



Let AB represent a plane sur- 

 face moving horizontally with 

 velocity v lf while a horizontal jet 

 moving with greater velocity v, 

 strikes it centrally. Let W be Fifl ' 50 



the weight of water acting on the 



surface per second. Then the stored work or kinetic energy 

 of the water 



and if the body were at rest, this would be the loss of 

 energy. 



From Anal. Mechs., Art. 208, (4), if m' be very great as 

 compared with m, the loss of kinetic energy by impact 

 becomes 



tmty-vt)'. (2) 



Hence, if we first suppose that the water after impact 

 moves on with the velocity of the body, we have by (2), 



W 

 work lost by impact = (v Vj) 8 r- (3) 



