The Buoyant Force

A rock feels lighter underwater because the water pushes back on it. That push is the buoyant force, and it exists because fluid pressure increases with depth. Picture a block submerged in water: the water pressing on its bottom face is deeper, so it is at higher pressure, than the water pressing on its top face. Pressure times area gives force, so the upward force on the bottom beats the downward force on the top. The difference is the buoyant force, and it always points up, opposing gravity.

Draw the free-body diagram of a fully submerged block and you get three forces: gravity pulling down (mg), the buoyant force pushing up (F_b), and, if something is holding the block in place, a tension or normal force. Archimedes' principle packages the pressure-difference argument into one clean rule: F_b equals the weight of the fluid displaced, or F_b = rho_fluid x g x V_submerged. Notice the object's own density and mass never appear. Only the volume it shoves out of the way and the density of the fluid doing the pushing matter. That is why a lead block and a wood block of identical volume feel identical buoyant forces underwater, even though lead sinks and wood floats. What changes between them is the net force: gravity dominates for lead, buoyancy dominates for wood, deciding whether each sinks, floats, or hangs in place.

This force is the bridge between fluid statics and the rest of the unit. The same pressure-versus-depth idea that builds F_b also explains why pressure gauges read differently at different depths, and comparing F_b to weight sets up floating equilibrium, where an object sinks just enough for the displaced fluid's weight to match its own.