Buoyancy

What is Buoyancy ?

• Buoyancy is the upwards force that fluids (liquids and gases) exert on objects which are immersed in them.

• We shall use the term ‘buoyancy force’ but the term ‘upthrust’ is commonly used as well.

Floating and Sinking

When an object is surrounded by a fluid, it will float if the buoyancy force is bigger than its weight.

object

weight

buoyancy force

If the weight is bigger than the buoyancy force, the object will sink.

What causes the buoyancy force ?

• Pressure increases with depth in a fluid.• So the pressure on the bottom of an object

immersed in a fluid is greater than the pressure on the top of the object.

• This means that the force pushing up on the bottom of the object is greater than the force pushing down on the top of the object.

• The extra upwards force is the buoyancy force.

Buoyancy in pictures

Pressure increases with depth

Pressure at the sides cancels because it is equal and opposite at all depths

Low pressure at top

Higher pressure at bottom

Buoyancy in pictures

Pressures cause a force down on the top and a force up on the bottom

Low pressure at top

Higher pressure at bottom

Buoyancy in pictures

Pressures cause a force down on the top and a force up on the bottom

Small force down on top

Bigger force up on bottom

Buoyancy in pictures

Buoyancy force

Bottom

force

Top force