Why Heavy Things Float: Archimedes Principle and Buoyant Force

What is buoyant force in physics? What does Archimedes’ principle state? 📢 Learn 11 Killer Tips to Nail Physics: ▶ https://thesciencecube.com/ 📢 Join our Telegram Channel: ▶https://t.me/TheScienceCube_Community This lesson explains the upward push from fluids, builds buoyant force from pressure differences, and uses Archimedes’ principle to decide when objects float, sink, or stay neutrally buoyant. What You’ll Learn 1. Explain how pressure increases with depth and creates a net upward buoyant force. 2. Use Archimedes’ principle to relate buoyant force to the weight of displaced fluid. 3. Calculate buoyant force with Fb = rho_fluid * V_displaced * g and Fb = m_fluid * g. 4. Decide if an object floats, sinks, or is neutrally buoyant using density comparisons (rho_object vs rho_fluid). 5. Find the fraction of an object’s volume that is submerged at equilibrium. 6. Draw clear fluid free-body diagrams (FBDs) and avoid common misconceptions about buoyancy. Key Concepts Covered buoyant force, Archimedes’ principle, displaced fluid, hydrostatic pressure, density and relative density (specific gravity), static equilibrium in fluids, fraction submerged, apparent weight in a fluid Prerequisite or Follow-Up Lessons Pressure vs. Depth and Pascal’s Law Density and Specific Gravity *Full Lesson Transcript * : Buoyant Force and Archimedes’ Principle: Why Things Float or Sink Hook: Even an elephant can float in water. How can something so heavy be supported instead of sinking? The key is an invisible upward push from the fluid called the buoyant force. Water-filled sack in water: Imagine a thin plastic sack completely filled with water and fully submerged in a larger tank. It stays in place, so the downward gravitational force is balanced by an upward force from the surrounding water. _Where buoyant force comes from_: Fluid pressure increases with depth. Pressure acting on each surface patch creates forces; the deeper bottom feels a larger upward push than the top feels downward. Horizontal forces cancel; vertical ones add to a net upward buoyant force. Equilibrium of the water sack: Because the sack is at rest, Fb equals the weight of the water occupying that volume: Fb = m_fluid g This means the buoyant force equals the weight of the water inside that same space. Replace with a stone (same shape and volume): Swap the water for a stone that fills exactly the same volume. The water around it cares only about the displaced volume, so the buoyant force is still the weight of the displaced water. But the stone’s weight is larger than Fb, so the net force is downward and it sinks. Replace with wood (same shape and volume): Put in a wood block instead. The buoyant force is unchanged (same displaced volume), but now the wood weighs less than Fb, so the net force is upward and it rises to the surface. This is why logs, ice cubes, and ships float. Archimedes’ principle Any body immersed in a fluid, fully or partially, experiences an upward buoyant force equal to the weight of the fluid displaced by the body. Fb = m_fluid g = rho_fluid V_displaced g Important: this depends on the displaced fluid, not the object’s own mass. Partial immersion and fraction submerged If a block floats with 40% of its volume under water, only that 40% counts for displacement and for Fb. The buoyant force comes from the weight of just the submerged portion of fluid. How floating finds its level Set a wood block on water. Gravity pulls it down; as it sinks a little it displaces more water, increasing Fb. It settles when Fb grows to match its weight. Floating condition: Fb = Fg Equivalently, a floating body displaces its own weight of fluid. _Float, sink, or neutral_: Float if rho_object less than rho_fluid (there is a submerged fraction where Fb = Fg). Sink if rho_object greater than rho_fluid (even fully submerged, Fb less than Fg). Neutral if rho_object = rho_fluid (it can hover at a depth). Golden rule and closing thought: If it floats, its weight equals the weight of fluid it displaces. Boats, hydrometers, and icebergs all obey this simple rule. Buoyancy shows how even very heavy things can be supported when they displace enough fluid to balance their weight. Who is this for Class 11 physics student, AP Physics students, students appearing for competitive exams like IIT JEE and NEET. Key Moments 0:00 What is buoyant force in physics? 1:10 How does pressure vs depth create buoyancy? 2:24 Why does wood float but stone sink? 4:34 What does Archimedes principle state? 5:43 When does an object float, sink, or stay neutrally buoyant? 📢 Watch the Full PLAYLIST Here: ▶    • Fluid Mechanics : Pressure, Buoyancy, and ...