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That would've been my first guess. Does it have to do with the rock at first either being out of the water (in the canoe) or tied to the canoe which is not completely submerged--thus the end result being the water level rising? Displacement obviously has something to do with it, no? NO-- wait wait... When the rock is in or attached to the canoe the canoe plus rock is considerably heavier and thus displaces more water over a wide area than when the rock is on the bottom, so the water level goes down. The rock's water displacement is very small compared to the conoe's. How's that?
In both instances the water level decreases slightly. The first because you're reducing the displacement of the canoe hull when you throw the rock overboard.
The 2nd is similar but the water level should decrease less because the weight of the rock in water will be slightly less as it's more buoyant (yes rocks are buoyant - just not very) so it is pulling the canoe hull down slightly less. It's less of a net change because the rock is already displacing it's volume in the water.
You have a canoe with a rock hanging off the front on a rope, completely submerged with water.
However, the way you wrote this - if the canoe itself is completely submerged in the water - the net change is zero...lol
Question of the day: Does God love me for some unexplained reason and blessed me with a wonderful wife - OR - Did God bless me with a wonderful wife and, therefore, has learned to love me for her sake?