May be an appeal to authority, but a guy in his 3rd year of physical sciences at Cambridge told us it was D, with a pretty good explanation
.
And yeah, it's not a trick or a riddle. It's just a question based on the physics laid out in front of you.
If the piece of ice+stone is floating, as the question states, then the weight of the ice+stone is equivalent to the weight of the displaced water. As the ice melts, there isn't a change in water level. For every gram of ice that melts, you're displacing one less gram of water, but another gram of water is added from the melting ice. In other words, no change. When the stone slips out of the ice and sinks, however, the ice becomes more buoyant, more of it being exposed to the air, causing the water level to fall to some extent.
In other words, D.
Yeah, that's basically correct.
The only thing is that we required a bit more explanation to see what exactly the stone was doing (your's is basically right, but kindof hard to follow if you don't already know how it works).
Basically, you first need to know that to float, something needs to displace its own weight in water.
So while the stone is in the ice, it is displacing an amount of water equivalent to its own
weight.
However, once it's out of the ice, it's only displacing an amount of water equivalent to its own
volume.
Since the stone must be more dense than water (because it sank) this means that its weight in water is greater than its volume in water. So the water level would lower when the stone is released.
It might be easier to think of it in some other form (we had "throwing gold bars out of a boat").
(hey, I know we're meant to be "back on topic", but it seems unfair to post a problem then not post the solution
)