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This problem was originally posted on another board by a former acquaintance of mine. That board no longer exists.

An ordinary rotary garden sprinkler has bent arms that spew out water, causing them to rotate as the water rushes out. Let's say that the sprinkler normally rotates clockwise. If the sprinkler was submersed in water instead and connected to a water pump, causing it to suck in water, how will the sprinkler behave?

To get full marks, please provide reasons for your answer.

From my meager understanding of what could possibly happen, the sprinkler would move in an opposite direction as to what it normally moves (if it is capable of moving that way mechanically). This would happen since the pump would create a vaccum within the sprikler and after the water that had at once filled the sprinkler been pulled into this vaccum, the external water would then be drawn in. Now if the sprinkler was fixed (non rotating) the water would enter normally and this question would be null and void, yet since it is capable of moving the force of the water being pulled from its sorrounding would create another vaccum as the water itself enters the sprinkler. This external vaccum wouldn't affect the water entering the sprinkler, but it will affect the sprinkler's nozzles (should it have more than one) and pulls them towards them.

If this is wrong or right can only be proven by putting it to the test. Which upon having my interest pique on the question I will preform once I gather the neccesary matterials.

isn't it possible that the sprinkler stops rotation? I theorize that this is because the pressure of the water as was much greater than the pressure of the air, which given the malleability of the sprinkler tended to make circular motion. This work originates from the pressure which is being thrown out of the sprinkler working agains the atmospheric pressure (which would be standard 1atm pressure), and thus displacing the arms (but because they are fixed and placed on a pivot) it is displacing them in a circular motion.

Submersed in water. The sucking motion comes from a vaccum inside the sprinkler, which, for being a vaccum would require no work to push against. So so the system is moving towards equilibrium (until the vaccum stops of course). Whereas in the first case it was moving against equlibrium (err... I'm afraid it's not making much sense).

grr, I feel something is not right about my idea of the source of the rotation in the first place, but I shall think about it some more.

It wouldn't move.

As soon as the pump is started it would move slightly towards, but while it's pumping steadily there would be no movement. I've seen this question before, I believe I just used conservation of angular momentum.

Working? Later razz

Actually, upon thinking, because the water has viscosity, I would go for slight acceleration opposite to the normal when it's expelling water.

My thoughts on this would be that the sprinkler would rotate in reverse, 'assisting' the draining water in filling the vacuum. But I don't know much of physics, so I very well could be wrong.

I'm not sure how to do that without almost giving it away, but alright: consider the effect of the water pressure gradient and the momentum of the inflowing water.

Sorry, no. Seriously, though, about one of the best ways to do this probablem would be to simply draw a diagram of a sprinkler arm and consider the aforementioned sources of force.

Assuming that it is submerged: Initially, the suction will result in an imbalance of pressure causing it to rotate in the direction of the water being sucked. If we are talking about the same ordinary rotary garden sprinkler, this is opposite the direction of spin - it moves counterclockwise. However, if the water's momentum balances the pressue gradient that is exerted on the opposite wall: the sprinkler will eventually stop moving.