*Originally posted by sugiezd*

**Any of you heard of Archimedes?
**

The rock will displace its own volume of water whether in the boat or on the bottom of the lake.

That is how density is calculated, you know the weight of an object, you measure the volueme of water it displaces, divide one by the other and there you have it.

Remember the eureka can?

The water level remains the same.

As mentioned above, this is wrong. Any submerged body will displace a certain volume of water, and one of two things will happen:

(1) If the density of the object is less than that of water, it will float, and the weight of the displaced water will equal the weight of the object.

(2) If the density of the object is greater than that of water, it will sink, and the weight of the displaced water will be less than the weight of the object. In this case, the object displaces the maximum amount of water it can, which is equal to the volume of the object.

Since the denser object reaches a maximum in the amount of water it can displace by itself, it's easy to see that if you can get it to float (by, say, putting it in a boat) it will displace more water. Therefore the water level will be higher if the rock is in the boat.