# Archimedes' Principle and Buoyancy Have you ever wondered why a massive steel ship floats, while a small pebble sinks? Or why a hot-air balloon rises, and a water-filled mug feels lighter when submerged? The answer to these questions lies in the concept of **buoyancy**, which is explained by Archimedes' Principle. This principle describes the upward force exerted by a fluid on any object submerged in it. ## Buoyant Force (Upthrust) When an object is submerged in a fluid (a liquid or a gas), the fluid exerts pressure on all surfaces of the object. Since pressure increases with depth, the pressure on the bottom surface of the object is greater than the pressure on its top surface. This pressure difference results in a net upward force on the object, known as the **buoyant force** or **upthrust**. ## Archimedes' Principle Archimedes' Principle gives us a way to calculate the magnitude of this buoyant force. **Statement:** *An object wholly or partially submerged in a fluid experiences an upward buoyant force equal to the weight of the fluid it displaces.* **Mathematical Formulation:** Consider a cylindrical object of height $h$ and cross-sectional area $A$ submerged in a fluid of density $\rho_f$. The depth of the top surface is $h_1$, and the bottom surface is $h_2$. - Pressure on the top surface: $P_1 = \rho_f g h_1$. The downward force is $F_1 = P_1 A$. - Pressure on the bottom surface: $P_2 = \rho_f g h_2$. The upward force is $F_2 = P_2 A$. - The net upward force (buoyant force, $F_B$) is the difference between these two forces: $$F_B = F_2 - F_1 = (P_2 - P_1)A = (\rho_f g h_2 - \rho_f g h_1)A$$ $$F_B = \rho_f g (h_2 - h_1)A$$ - Since $(h_2 - h_1)A$ is the volume of the submerged part of the object ($V_{sub}$), which is also the volume of the fluid displaced, the formula becomes: $$F_B = \rho_f V_{sub} g$$ This is the weight of the fluid displaced. ## The Principle of Flotation Whether an object floats, sinks, or remains suspended depends on the balance between its weight ($W$) and the buoyant force ($F_B$). | Condition | Comparison | Behavior of the Object | | :--- | :--- | :--- | | **Sinks** | Weight > Buoyant Force ($W > F_B$) | The object accelerates downwards. | | **Floats** | Weight < Buoyant Force ($W < F_B$) | The object rises until the buoyant force on the submerged part equals its weight. | | **Suspended** | Weight = Buoyant Force ($W = F_B$) | The object remains in equilibrium at any depth. | **Law of Flotation:** A floating object displaces a weight of fluid equal to its own weight. ## Applications ### Ships A ship is made of dense steel, but its hollow shape displaces a vast volume of water. This creates a buoyant force large enough to support the ship's weight, allowing it to float. A ship sinks if it takes on too much weight (from cargo or water), causing its total weight to exceed the maximum buoyant force. ### Submarines Submarines control their depth by altering their weight. They have ballast tanks that can be filled with water to increase their overall weight, causing them to sink. To rise, compressed air is used to force the water out of the tanks, decreasing their weight and allowing the buoyant force to push them to the surface. ### Hot-Air Balloons A hot-air balloon rises because the hot air inside it is less dense than the cooler, surrounding air. The balloon displaces a large volume of the cooler, denser air, creating a buoyant force greater than the balloon's total weight. ## Practical Questions and Answers **Q:** Why does a mug filled with water feel lighter underwater? **A:** When the mug is underwater, it is supported by an upward buoyant force from the surrounding water. This buoyant force counteracts some of the mug's weight, so you only need to apply a smaller force to hold it up, making it feel lighter. This is often referred to as the "apparent weight" of the object. **Q:** How can a ship made of iron float, when an iron nail sinks? **A:** The key is density and shape. A nail is a solid piece of iron, which is much denser than water, so it sinks. A ship, however, is shaped to be mostly hollow. Its large volume displaces a huge amount of water, and its *average density* (the total mass of the ship divided by the total volume it occupies) is less than the density of water, allowing it to float.

Upthrust And Archimedes Principle | Fluid Mechanics | Physics 11