Why do objects float?

A surprising observation

Place an iron paperclip on the surface of the water: it sinks immediately. Yet a huge steel ship, which is much heavier, floats effortlessly on the ocean. How can this paradox be explained?

The answer lies in a physical phenomenon: when an object is immersed in a fluid (water, oil, air, etc.), that fluid exerts an upward force on it. This force is known as Archimedes’ principle.

The bath experiment

Legend has it that Archimedes, an ancient Greek scientist, realised this phenomenon whilst taking a bath. As he submerged himself, he noticed that the water level rose and that he felt lighter. He is said to have then run through the streets shouting “Eureka!” (“I’ve found it!”).

Floating or sinking: some examples

Object	Material	Does it float or sink?
Paper clip	Solid iron	Sinks
Container ship	Steel (hollow)	Floats
Ice cube	Ice	Floats (in water)
Coin	Metal	Sinks
Buoy	Plastic + air	Floats

Key takeaways

It is not just the weight of the object that matters, but also its shape and the volume it occupies in the fluid. A boat floats because its hollow shape displaces a large volume of water, which creates significant buoyancy. In the next lesson, we’ll give a precise formula for this buoyancy.