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quantum quack: in a river I place a rock that deflects the fast flowing water, is the rock doing work?
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Let's see. Since the flow of water is being diverted, then the water molecules are having their direction of motion changed ... by the rock. A change in direction is an acceleration, so the rock is causing the water molecules to accelerate. And it takes a force to cause acceleration, so the rock must be exerting a force against the water molecules.
This is a reaction force, according to Newton's third law (for every action force there is an equal and opposite reaction force). The water is in motion and is striking the rock, exerting an action force against it (but doing no observable work, since the rock doesn't move to any measurable degree). The rock is in turn exerting a reaction force against the water molecules.
Since the molecules of water are in a fluid state, they can be considered to be nearly independent: at least more independent than the molecules that make up the solid rock, which all act as a single unit. So it is the water molecules that experience the (greater) acceleration.
So far, we have the water molecules exerting a force against the rock but doing no work (since the rock doesn't move), and the rock exerting a reaction force against the water molecules and causing them to accelerate.
That all seems straightforward. But the original question still remains unanswered: does what the rock is doing count as work?
As alluded to above, work requires not just a force being exerted against an object, but also for the object having the force exerted against it to be moved a distance as a result of the force. Now, the rock is passively deflecting the water molecules off into a different direction - it is accelerating them by means of a reaction force - but does that actually count as moving the water molecules through a distance because of the force exerted on them? I am not sure.
