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Originally Posted by quantum quack
in a river I place a rock that deflects the fast flowing water, is the rock doing work?
if so how is it's energy use conserved?
the question is really about how a seemingly static object can perform work or at least a function like deflecting a torrent of water with out actually expending any recordable energy. Yet it provides a force that deflects the energy of the water [in this example]
where does the object get the energy from to deflect water?
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Yes, the rock is doing work.
The result of this work is the ripples of water you'd see trailing the rock, much like a boat's wake.
The rock itself is not the agent of the work being done, but is just there, by accident, so to speak. The actual effort is in pressure from the water trying to get the rock to overcome the friction between itself and the riverbed.
The friction between the rock and the the riverbed depends on the weight of the rock, and it's texture. If the rock was very light, the stream won't have much of an effort to wash it away. But being a normal rock, I assume it would be considerably heavier than water, which means you need a fairly fast flowing stream to push it. Unless both the riverbed and the rock are very,
very smooth - then once again the friction will be reduced.
Now - say the rock is so heavy and coarse so as not to move at all? Is there any work being done?
Yes.
But not readily visible, and actually quite abstract. The force of the water pushing against the rock (of which the ripples is the evidence) is transmitted to the riverbed, which is fixed to the Earth's crust. So, in effect, the small stream pushing against the small rock is having an effect on the Earth itself. You might say that in ten kazillion years, when the Earth comes to an end, the (floating) continental plate experiencing the effect of your rock, will be two inches away from where it would have been, had you not thrown a rock in the stream.
Case in point - it has been calculated that the planet Jupiter will in 4 billion years be two feet short of where it was supposed to be, had Voyager 2 not stolen some of its angular momentum on its flyby in the eighties. Amazing.
