Why Suction Doesn’t Reverse the Spin
A sprinkler turns because its curved arms redirect moving water. With water flowing outward, the arms send it off in a sideways direction. The water leaves carrying angular momentum—the rotational equivalent of “oomph”—and the arms feel a reaction in the opposite direction, like a tiny rocket pushing itself forward by ejecting gas.
Suction reverses the water’s route, but it does not simply reverse that push. Water entering the arm openings already has a sideways component of motion relative to the sprinkler. As the curved passages guide it inward, they transfer angular momentum between the water and the arms. That exchange offsets much of the turning effect people expect from reversing the flow.
The Missing Piece Is the Water’s Motion
It is tempting to picture incoming water as moving straight into each arm, as if it had no rotational motion to give or take. But the arm openings are angled, and the water must follow those bends. The sprinkler therefore has to redirect the incoming flow, just as it redirects outgoing water. The resulting forces depend on how the flow changes direction—not merely on whether it points inward or outward.
In an idealized comparison, the inward flow’s angular-momentum exchange largely balances the torque that would make the sprinkler spin the other way. Real sprinklers can still twitch or turn because of friction, uneven flow, and small differences in their shape; they just do not behave like a simple motor running in reverse.
The useful lesson is that a jet’s push is not determined by its direction alone: what matters is how the water’s rotational motion changes as it passes through the arms.
