Buoyancy

Buoyancy is the upward force that allows things to float in water or other fluids. When you put an object in water, the water pushes back against it. This push is called the buoyant force. It works opposite to gravity, which pulls objects downward. If the upward push of buoyancy is stronger than the object’s weight, the object floats. If the object is heavier, it sinks.

The Greek scientist Archimedes discovered the rule of buoyancy more than 2,000 years ago. He found that when an object is placed in water, it pushes water out of the way. The amount of water pushed away, or displaced, helps decide whether the object will float or sink. This is known as Archimedes’ principle, which says that the upward buoyant force equals the weight of the fluid the object displaces.

You can see buoyancy at work in everyday life. A wooden boat floats because it pushes away a large amount of water while weighing less than that water. A metal ship also floats for the same reason—it is shaped so that air fills most of it, making its overall density less than water. On the other hand, a rock sinks because it is denser and heavier than the water it displaces.

Buoyancy does not only happen in water. It also occurs in air. For example, a hot-air balloon rises because the warm air inside is lighter than the cooler air around it. Similarly, helium balloons float because helium gas is less dense than air.

Understanding buoyancy helps engineers design boats, submarines, and even airplanes. Submarines, for instance, can rise or sink by changing how much water they hold inside special tanks. This changes their density and the balance between weight and buoyant force. Buoyancy also plays a role in swimming, diving, and understanding how fish stay at certain depths using their swim bladders.

Buoyancy shows how forces work together in nature. It helps explain why some things float and others sink—and why life on earth depends on the balance between gravity and the push of fluids.