One solution to that problem is that when the rock is on the bottom of the lake, it exerts more force on that part of the bottom of the lake than is exerted at other places. By contrast, when the rock is still in the boat, the only thing touching the bottom of the lake is water, and the water pressure is the same everywhere, so the weight of the rock is distributed evenly across the entire lake.
This confuses me because it sounds like the situation “rock is fully submerged and sinking but still near the top of the lake” would be analysed like the situation “rock is on boat”, not “rock is on bottom of lake”. But that would give the wrong answer.
Well, it’s nonequilibrium, so pressure isn’t even at each layer of water any more...
When I picture this happening, there’s a pulse of high-pressure water below the rock. If you froze the rock’s motion while keeping its force on the water below it, I think the pulse would eventually equilibrate out of existence as water flowed to the side? Or if I imagine a fluid with strong drag forces on the rock, but which flows smoothly itself, it again seems plausible that the pressure equilibrates at the bottom.
(More confident in the first para than the second one.)
Thanks! “It’s nonequilibrium” feels like it points at my specific mistake. Apparently my intuitions don’t currently always remember to consider that question.
This confuses me because it sounds like the situation “rock is fully submerged and sinking but still near the top of the lake” would be analysed like the situation “rock is on boat”, not “rock is on bottom of lake”. But that would give the wrong answer.
What am I missing?
Well, it’s nonequilibrium, so pressure isn’t even at each layer of water any more...
When I picture this happening, there’s a pulse of high-pressure water below the rock. If you froze the rock’s motion while keeping its force on the water below it, I think the pulse would eventually equilibrate out of existence as water flowed to the side? Or if I imagine a fluid with strong drag forces on the rock, but which flows smoothly itself, it again seems plausible that the pressure equilibrates at the bottom.
(More confident in the first para than the second one.)
Thanks! “It’s nonequilibrium” feels like it points at my specific mistake. Apparently my intuitions don’t currently always remember to consider that question.