Solving Turbine Force: Vo = +21 m/s, Vf = -21 m/s, 27.2 kg/s

[mikefitz]

A stream of water strikes a stationary turbine blade, as the drawing illustrates. The incident water stream has a velocity of vo=+21 m/s, while the exiting stream has a velocity of vf=-21 m/s. The mass of water per second that strikes the blade is 27.2 kg/s. Find the magnitude of the average force exerted on the water by the blade.

(There is a pic of a turbine forcing water to make a 90 degree angle).

Wouldn't it be correct to say that the force by the water on the blade is equal to the force on the blade by the water due to Newton's 3rd law?

so just take 21 * 27.2 = 571.2? Why is this wrong according to my book?

 

[mikefitz]

for clarity, here is the image:

http://img167.imageshack.us/img167/4493/c7p11avk8.th.jpg

 

[3trQN]

wouldnt it be 2(21)(27.2) because final-initial = -21-21=-42...