A boat containing a heavy anchor floats in a reservoir. If the anchor is thrown overboard and is completely submerged, what happens to the water level in the reservoir?
When submerged the anchor displaces a volume of water equal to its own volume. When in the boat, the anchor is associated with displacing a volume of water with a weight equal to its own weight - this is a larger volume than its own volume because the anchor's density is higher than water's. Because the anchor displaces less water when it is submerged, the water level drops when the anchor is thrown overboard.