BUOYANCY BUOYANCY Upward net force on objects within

BUOYANCY

BUOYANCY • Upward net force on objects within a liquid • Occurs because pressure in a fluid increases with depth • Since the bottom is deeper, more force is pushing up than force on the bottom pushing down • Net upward force pushes objects up

ARCHIMEDES PRINCIPLE • The buoyant force on an object is equal to the weight of the fluid displaced by the object FB = Fw = mg ρ = M/V M = ρV FB = ρVg It is NOT the object that exerts the buoyant force, it is the fluid that the object displaces that causes it FB = ρfluidg. Vsubmerged

Sink or Float • The difference between the buoyant force and the object’s weight determines whether an object sinks or floats.

WHAT IS THE BUOYANT FORCE ON THIS OBJECT?

EXAMPLE • A 70 kg statue lies at the bottom of the sea. Its volume is 3. 0 x 10 -2 m 3. How much force is needed to lift it? • When a crown of mass 14. 7 kg is submerged in water, an accurate scale reads only 13. 4 kg. Is the crown gold?

FLOATING • An object floats on a fluid if its density is less than that of the fluid • When floating FB = FW • ρf. Vdispg = ρo. Vog • ρf. Vdisp = ρo. Vo • Fraction of an object submerged is given by the ratio of the density of the object to the fluid

EXAMPLE • A rectangular block of wood (1. 0 m x 2. 0 m x 0. 25 m) floats so that 0. 15 cm of its height is above the water level. a) find the density of the wood b) find the mass of the wood