What it shows

Does the level of the ocean rise or fall when a cannon ball is tossed overboard? A question of displacement.

How it works

A difficult effect to see at sea, but it becomes clear by taking some parameters to extremes. Reducing the ocean to 12L in volume, and the boat to practically no mass by using a plastic bowl, a cannon ball of 1kg mass suddenly becomes substantial. With the cannon ball in the boat, its weight is distributed throughout the boat; the lowered density increases the amount of water displaced (by the fraction of the boat submerged), raising the water level. When the cannon ball leaves the boat, the boat's displacement is very small because its density is now so low little of it is below the water line. The cannon ball itself, now at the bottom of the sea and so completely submerged, has a high density, so its displacement is small. The water level drops by several cm.

figure 1. throwing the cannon ball overboard

Setting it up

The ocean is contained in a 12L Plexiglas tank 30cm deep. The ship is a 6 cup plastic kitchen mixing bowl. A brass 1kg mass sits nicely, and you may want to attach a piece of string to it, so that it can be lowered instead of dropped overboard, and is easier to recover. Water levels can be marked with tape arrows.

Comments

The ocean is contained in a 12L Plexiglas tank 30cm deep. The ship is a 6 cup plastic kitchen mixing bowl. A brass 1kg mass sits nicely, and you may want to attach a piece of string to it, so that it can be lowered instead of dropped overboard, and is easier to recover. Water levels can be marked with tape arrows.