IMPACT OF JETS 



307 



II 



1 



applying the equation of momentum either may be considered. In any 

 other case, however, the change of momentum must be measured by 

 the change of absolute and not of relative 

 velocity. \\ 



If friction be neglected and also losses due 

 to shock, the velocity relative to the surface 

 will be unaffected by the impact. Also, the 

 pressure exerted on the surface at any point 

 will be in the direction of the normal at that 

 point. On these assumptions we may consider 

 the following cases : * 



(1) Normal Impact on a Stationary Plane Sur- 

 face (Fig. 163). 



Let A = sectional area of jet in square feet. 

 v = velocity of jet in feet per second. 



Then the weight of water impinging on 

 the plane per second = W A v Ibs., where 

 W is the weight of 1 cubic foot of water. 



The initial momentum of this per second normal to the plane 



W A v' 2 .. 



= - -ft. Ib. units. 

 9 



Since the final velocity is tangential to the plane, the final momentum 

 normal to the plane = 0. 



W Av z 

 .'. Change of momentum per second normal to plane 



*/ 



.'. Normal pressure on plane 



W A t- a n 



Ibs. 



9 



If 6 be the angle which the 

 sheet of water makes with the 

 plane of the surface on leaving, 

 the final velocity per second nor- 

 mal to the plane = v sin 0, and 

 the momentum in this direction 



FIG. 163. 



W A v' 



sin 0. 



FIG. 164. 



The change of momentum 

 per second and therefore the pressure normal to the plane is now 



W A ?; 2 

 9 



(1 - sin 6) Ibs. 



