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The speed of water is v before and after the water hits the blade. This
simply means the water changes direction (velocity), but not speed like a
tennis ball bouncing off a wall.
The kinetic energy of the water would be the same before and after the
collision. By conservation of energy, this means the turbine does not move.
By conservation of momentum, the momentum change of the turbine plust the
momentum change of the water is zero. This means the turbine does not have a
zero change of momentum.
The only way both of these conditions can be satisfied is if the turbine
mass approached infinite in comparison to the water mass. So there is
something unphysical about these constraints. Would one really build a
turbine which turned this slowly? I think not.