Buoyancy is a natural phenomenon that explains why an object floats, sink, or remains suspended when placed in a fluid such as water or air. At the heart of this behavior is the buoyant force, an upward force that acts on any object that is partially or fully submerged in a fluid.
According to Archimedes’ principle, this upward push is equal to the weight of the fluid displaced by the object. Because pressure and gravity act differently on the top and bottom surfaces, a net upward force is created. Whether an object floats or sinks depends on its density compared to the fluid and the weight of the fluid it can displace.
The Buoyant Force Formula
The buoyant force equals the weight of the fluid displaced by an object, a relationship first described by Archimedes. In simple terms, when a submerged object is placed in a liquid or gas, the surrounding fluid begins to exert pressure on its surface. Because pressure increases with depth, the pressure exerted on the bottom of an object is greater than the pressure acting downward on its top. This difference creates an upward buoyant force.
In physics form, the formula is written as F b = ρ V g, where ρ represents the fluid’s density (such as the density of water), V is the volume of water displaced, and g is the gravitational acceleration.
The magnitude of this force determines whether an object will float or sink. If the object’s density is less than the surrounding fluid, making it less dense the buoyant force can overcome the downward weight. This principle explains how a submarine adjusts its depth and how a balloon rises through a gas-filled environment.
How Density Determines Whether an Object Floats or Sinks
Whether an object floats or becomes one of the objects that sink depends on the balance of forces acting on it and the density of an object compared to the surrounding fluid. When an object immersed in a fluid experiences the force on the bottom being greater than the force on the top, an upward buoyant force is created. This happens because the pressure at the bottom is higher, producing a net upward push that acts against the downward force of the object’s weight and the gravitational force pulling it down.
If the buoyant force is greater than the weight of the object, the force on the object allows the object to float. This is why a large ship, despite its mass, becomes a floating object—its average density is lower than the water it displaces. In contrast, when the buoyant force is smaller than the weight of the object, it will sink.
A clear example is how salt water makes it easier to float than fresh water because it increases buoyant force. The same principle explains how a hot air balloon rises, even though it is not immersed in water, and why the apparent weight of an object changes when immersed in water.
Master Buoyancy & Buoyant Force
Learn how buoyancy and buoyant force work in fluids with clear explanations and real-world examples that make Archimedes’ principle easy to understand.
Real-World Applications of Buoyant Force
The principles of buoyancy apply to almost every object in a fluid, regardless of size or shape. The force on any object immersed in a fluid is always directed upward and acts perpendicular to the object’s surface. Whether a floating object stays on the surface, sinks, or rises depends on the tendency of an object to displace enough fluid to balance its weight. In simple terms, whether the object floats is determined by how much displaced fluid it creates and its overall density compared to the surrounding substance.
A submarine is a classic example. By filling or emptying its ballast tanks, it controls how much water it displaces. When water fills the tanks, the submarine becomes denser than the fluid around it, the buoyant force is less than its weight, and it begins to descend to a greater depth. When air is pumped back in, the submarine displaces more water, allowing it to float or rise.
The same logic explains why a ship floats despite its heavy mass. Even when completely submerged at launch, its shape ensures the volume of fluid is greater than the ship’s average density would suggest, creating enough buoyant force. In marine life, a fish uses a swim bladder to remain suspended by adjusting the amount of gas inside, carefully balancing buoyant force without constant movement.
Buoyant Force in Completely Submerged Objects
When an object is completely submerged in a fluid, multiple forces act on it at the same time. The most obvious is the gravitational force, which pulls the submerged object downward due to its weight. Opposing this is the buoyant force, an upward force created by pressure differences within the fluid. As pressure increases with depth, the fluid pushes more strongly on the bottom of the object than on the top, causing the buoyant force to act upward.
If the buoyant force is less than the object’s weight, the downward force dominates and the object will sink. When the buoyant force becomes greater than the weight of the object, the upward force wins and the object rises. In cases where the two forces are equal, the object remains suspended at a fixed depth. Understanding how each force acts on a completely submerged object helps explain why some objects sink rapidly, while others hover or slowly rise within a fluid.
Conclusion
Archimedes’ principle provides a clear foundation for understanding buoyancy and how buoyant force works in everyday situations. By calculating buoyant force as the weight of the fluid displaced, we can predict whether the object floats, sinks, or remains suspended. This upward force acts on any object placed in a fluid and depends largely on density and displacement.
From ships and submarines to balloons and swimming, these concepts appear all around us. For students looking to strengthen their understanding of physics topics like this, Your Private Tutors can offer personalized guidance to make complex ideas easier to grasp.
