When an object is placed in a fluid, it either floats or sinks. All objects in a fluid experience a buoyant force. For example, a metal ball sinks, while a rubber ball floats. Similarly, a submarine can sink and float by adjusting its buoyancy. The concept of buoyancy raises several interesting questions. For instance, where does this buoyant force come from? How much buoyant force is required to make an object sink or float? Do objects that sink get any support at all from the fluid?
To get an insight into these questions, we need to understand how fluid pressure varies with depth. The weight of a fluid increases with an increase in depth; thus, the pressure also increases. This increase in pressure creates a difference in pressure at the top and the bottom surface of the object, leading to an upward force known as the buoyant force. If the buoyant force is greater than the object's weight, the object rises to the surface and floats. If the buoyant force is less than the object's weight, the object sinks. If the buoyant force is equal to the object's weight, the object remains suspended at that particular depth. The buoyant force is always present, whether the object floats, sinks, or remains suspended in the fluid.
This text is adapted from Openstax, University Physics Volume 1, Section 14.4: Archimedes’ Principle and Buoyancy.
In a fluid, the pressure increases with depth due to an increase in the weight of the fluid pushing it down from above.
Consider a fluid in a container. With multiple holes on its walls at different heights, fluid from each hole will follow a different trajectory.
The greater the pressure, the farther the fluid falls from the container.
Thus, an object in a fluid experiences a greater pressure at its bottom than at its top.
This pressure difference produces a net upward force on the object known as the buoyant force.
If the buoyant force on a completely submerged object is greater than its weight, it rises to the surface and floats.
When the object protrudes above the surface of the fluid, its buoyant force decreases because the air at the top exerts a lower pressure than the water at the bottom. Since its buoyant force is equal to its weight, it floats.
If the buoyant force is less than its weight, the object sinks.
If the buoyant force equals the object’s weight, the object remains suspended at that depth in the fluid.
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