Buoyancy is the upward force exerted by a fluid that opposes the weight of an immersed object. This force arises due to the pressure difference of the fluid acting on the submerged and exposed parts of the object. Archimedes' principle states that the buoyant force on an object equals the weight of the fluid it displaces. An object will float if its density is less than the density of the fluid it displaces.

1. BUOYANCY

2. 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

4. 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

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

6. WHAT IS THE BUOYANT FORCE
ON THIS OBJECT?

7. 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?

8. 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

9. 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