BUOYANCY
A S H O R T L E S S O N O N W H Y T H I N G S F L O AT
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 = ρfluidgVsubmerged
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 70kg statue lies at the bottom of the sea. Its volume is 3.0 x 10-2 m3. How much force is
needed to lift it?
• When a crown of mass 14.7kg is submerged in water, an accurate scale reads only 13.4kg. 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
• ρfVdispg = ρoVog
• ρfVdisp = ρoVo
• 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 (1.0m x 2.0m x 0.25m) 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