Rock

Two men put a large rock into a rowing boat & take it to the middle of a large pond. They then throw it over the side.
How does the water level change?

My 8th grade science teacher tried to tell us that when you sink something it water, it will displace the weight of water equal to it's weight. She was wrong. This displacement rule only comes into effect if something is floating.

The water will not rise the moment the rock is dropped. It rose when the boat, rock, and men entered the lake, and will sink again when the men and boat exit the lake. The rock's displacement will remain, as it is now at the bottom of the lake. No level will change at the moment the rock is removed from boat and placed in lake.

close, but no cigar

While in the boat, the rock displaces a volume of water that has the same weight as the rock. Once in the water, the rock only displaces water equal to its volume, which is less (assuming the 'rock actually' sinks, of course). Hence, when the rock is in the water, the water level will be lower (less upwards displacement).

Originally posted by Mephisto2
While in the boat, the rock displaces a volume of water that has the same weight as the rock. Once in the water, the rock only displaces water equal to its volume, which is less (assuming the 'rock actually' sinks, of course). Hence, when the rock is in the water, the water level will be lower (less upwards displacement).

OH. right. I answered it in my first part sorta, then went on to claim the wrong answer.

Originally posted by Mephisto2
While in the boat, the rock displaces a volume of water that has the same weight as the rock. Once in the water, the rock only displaces water equal to its volume, which is less (assuming the 'rock actually' sinks, of course). Hence, when the rock is in the water, the water level will be lower (less upwards displacement).

This is correct. Imagine you had a grain of sand in the boat that weighed 100kg. In the boat it really pushes the boat down (and hence water up) whereas on the bottom of the pond its effect is negligible.

Originally posted by SPMars
This is correct. Imagine you had a grain of sand in the boat that weighed 100kg. In the boat it really pushes the boat down (and hence water up) whereas on the bottom of the pond its effect is negligible.

Alien sand?

Originally posted by SPMars
This is correct. Imagine you had a grain of sand in the boat that weighed 100kg. In the boat it really pushes the boat down (and hence water up) whereas on the bottom of the pond its effect is negligible.

actually I think it would be quite likely to cut through the boat before you got it out there, being so much mass on such a consentrated point. 😛