# When discarding water in a cup (answer)

## Let's take a look!

## What kind of experiment is this?

## Experimental procedure and explanation:

- The answer is "the height of the water surface does not change".
- If you remove the water in the cup, the floating island will be lifted (virtually) by the same amount of water weight lost from the cup, and a cavity will be created.
- The volume of this cavity is the same as the volume of water in the cup (because both weigh the same), so the cavity is exactly filled. Therefore, the height of the water surface does not change.

[Keywords] | Buoyancy |

[Related items] | When ice on a floating island melts |

[Reference] | “The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs, P45-46. “Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, P188-189. |

Last Update：4.13.2021