You are in a boat on a fresh water lake. In your hand is a rock. You throw the rock into the lake. How is the?

Question

How is the water level affected?

Answer 1

It depends on the size of the rock, but in any case the water level rises.

Answer 2

I would guess it increases because the weight you displace on top of the water would be less than something that completely submerges.  So although you have less weight displacing on top you have more in the water raising it slightly above what it was but probably very minimal.

Answer 3

First of all, let's state some assumptions and knowns:

1)  Everything in the boat will displace a given amount of water.  The boat will sit IN the water, not ON the water.  This displacement is based on the mass of everything inside the boat and the mass of water that's displaced.

2)  Let's give a mass to the rock.  Let's assume it weighs seven pounds.  This is fairly massive, yet easy for anyone to pick up and place in a boat.

3)  Water weighs almost 7 pounds per gallon.  Not very exact, but let's use this number for assumptions and make the math easier.  This means that the rock weight in the boat displaces one gallon of water.

4)  Rocks are denser than water....that's why they sink.  Most rocks have a density of about 2 g/cm3 or.....20 lb/gallon.  

5)  Throwing the rock into the water will now lessen the boat's water displacement by 1 gallon (the boat weighs seven pounds less).  Thus, the boat rises.  The water level falls on shore by the displacement of one gallon of water.

6)  The rock sinks in the water.  The rock displaces 7/20 of a gallon of water in the pond/lake.  This value is just about a third of a gallon (7/21 would be a third).  This causes the water level at shore to rise by about 1/3 of a gallon.

Add/Subtract the two effects above and you'd get a fall in water level by almost 2/3 of a gallon of water displacement.