Hollow hull and solid marble

Open to everyone until Wednesday, September 30

To keep it, print it while it is open.

A ship’s hull is mostly air-filled space, so the whole ship pushes aside more water than it weighs. A solid glass marble has no air inside it, so its density stays higher than water and it sinks.

In the picture

Air chambers inside the hull: empty spaces that hold only air
Steel hull plating: thin metal walls around the chambers
Solid glass marble: no air inside, resting on the tank floor
Water in the tank: the liquid the marble sinks through

Examples

A steel cargo ship floats across the ocean.
A glass marble dropped into a glass of water sinks to the bottom.

Key takeaways

Average density is total mass divided by total volume, and trapped air adds volume without adding much mass.
An object floats when its average density is less than the density of the water around it.