Cannon Ball Boat Puzzler
Archimedes' principle - buoyancy - displacement
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 Plexiglass 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 Plexiglass 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.