There is a pipe which is bent in the shape of a right angle as shown. The cross section area of the pipe is A, the velocity of the water inside is v, and the density of water is ρ. The momentum flowing per second towards the right corner is thus: Av2ρ towards right and momentum flowing per second downwards from the corner is Av2ρ downwards. Then why is the force acting on the liquid the vector difference of the flows of momentum per second? Isn't force equal to change in momentum per unit time? But we never calculated change in momentum, we calculated change in flow of momentum per unit time. This leaves me confused.