1. The problem statement, all variables and given/known data Here it is http://imgur.com/a/Wltb0 2. Relevant equations F=dP/dt Cosine rule 3. The attempt at a solution For the first part of the question i'm not exactly sure what the resultant of these vectors represent. First i thought they represented the relative speed of water to the blades but thats only for the first diagram. For the second Part i got the change in momentum to be (let m= mass per unit time here) = mt(Vi*cos(alpha)+Vo*cos(beta)). Now i assumed that collision is elastic and so the relative speed of approach and relative speed of separation should be the same. this gave me Vi^2 + Vb^2-2Vb*Vi Cos(alpha)=Vo^2+Vb^2-2Vo*Vb*cos(B). from these I could not get the required result. However now that i'm thinking about it i think that the first vector sum represents the relative speed of approach and the second one represents the relative speed of sepration and i think the left hand side of the equation should thus be the same thing except Cos(alpha) should be replaced by Cos(pi-alpha) that would give me the result. But was that my mistake ?