Fluid Mechanics

[michaelboy]

Imagine you're floating in a boat in a small pond. There is a water-height marking stick in the pond. You have a large heavy rock on board and you decide to push the rock in the water. The rock sinks to the bottom. What then happens to the water level shown on the stick? Does it go up, down or stay the same?

Comments

[keplers_angels]

I want to say it stays the same.

Because I like the idea of conservation in this system. It feels good. BUT, I'm not sure if it actually has something to do with the shape of the boat-- volume displaced by the submerged part... and I am assuming that the bottom of the pond is considered closed --like a lined pool sort of deal...maybe the distinction between mass and volume is what's tricky about this... maybe the water level is not conserved?

What is the answer?

[michaelboy]

There's no secret to the boat or pond and a lined-pool would yield the same result. Any boat will displace a volume equal to the equivalent weight of water. So since a rock is more dense than water, the volume it will displace is larger than the heavier rock. Once you toss it in the water the rock will only displace its own volume (which ends up being less than the equivalent water volume), therefore the water level will do down.