Archimedes' Principle

Archimedes’ Principle says that a liquid pushes an object upward with a force equal to the weight of the liquid the object displaces (pushes aside).

• When you place an object in water, it takes up space and forces some water out of the way. The more water it pushes aside, the greater the upward buoyant force.
• Imagine lowering a plastic bowl upside down into water. Water cannot fill the space inside the bowl, so the bowl pushes aside a large amount of water. This produces a strong upward push.
• A huge steel ship can float because its hollow shape displaces a large volume of water. Although steel is heavy, the ship’s overall average density is less than that of water.
• A solid coin displaces only a small amount of water, so the upward push is too small to support its weight. It sinks.
• This principle helps engineers design boats, submarines, life jackets, and even hydrometers, which measure how dense a liquid is.

Floating is therefore not mainly about an object being large or small—it depends on how its weight compares with the weight of the liquid it pushes aside.