Archimedes' Principle

When an object is placed in a fluid, the pressure is greater at deeper points, so the bottom of the object is pushed up more strongly than the top is pushed down. That pressure difference adds up to a net upward force, and for a fully or partially submerged object it equals the weight of the fluid that the object displaces. The formula F_b = rho g V shows that this buoyant force depends on the fluid’s density, gravity, and the volume of fluid displaced, not directly on the object’s own mass.

Use this formula whenever you need the upward buoyant force on an object in a liquid or gas, such as finding whether something will float, estimating apparent weight underwater, or calculating how much lift a submerged or floating body gets. It is also useful in density and displacement problems in ship design, hydrometers, and lab measurements.

• Using the object’s own volume instead of the displaced fluid volume
• Forgetting to use the density of the fluid, not the density of the object
• Mixing units, such as using cm^3 with kg/m^3 without converting to m^3