Archimedes' Principle

Over 2,000 years ago, a Greek scientist named Archimedes made an important discovery about floating. He found that when an object is placed in water, the water pushes it up with a force equal to the weight of the water the object displaces. This explains why some things float and others sink.

The Principle Explained

When you put an object in water, it pushes some water out of the way. The water pushes back with an upward force called buoyancy. If the buoyant force is greater than the object's weight, it floats. If the object weighs more than the water it pushes aside, it sinks.

Why It Matters

Archimedes' Principle helps engineers design ships and submarines. A steel ship floats because its hull is hollow and pushes aside a huge amount of water. Submarines can sink or float by filling or emptying tanks with water. Hot air balloons also work on this principle, but with air instead of water.

Fun Facts

• A helium balloon floats because helium is lighter than the air it displaces.
• The Dead Sea is so salty and dense that people float in it without even trying.
• Archimedes used his principle to prove that a king's crown was not made of pure gold.

Did You Know?

The story says Archimedes discovered his principle while getting into a bath. He noticed the water level rose and realized the key to buoyancy. He was so excited he supposedly ran through the streets shouting Eureka!