# Fluid displacement

1. Jan 21, 2007

### vu10758

1. The problem statement, all variables and given/known data
A small child floats in a boat in a swimming pool. The boat contains an anchor
The child drops the metal anchor into the water and watches it sink to the bottom. Meanwhile, the water level in the pool (measured by its height on the side of the pool) actually drops. Why?

2. Relevant equations

3. The attempt at a solution

I am thinking that if the anchor is dropped into the water, the water should rise because of Archimedes principle. However, I am told that the water level actually drop. Where did the water go? Is it because as the boat pushes down on the water, the water pushes the boat further up due to Newton's third law, causing it to appear as if the water level has dropped? Or is the reason something else? I know that the change in pressure is the same as the weight of the boat so does this have anything to do with the question?

2. Jan 21, 2007

### Kurdt

Staff Emeritus
Think about the mass of the anchor and its volume. When the anchor is in the water what volume of water does it displace? When the anchor is in the boat what volume of water does it displace?