1. Buoyancy

2. 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.

3. Floating and Sinking
When an object is surrounded by a fluid, it will float if the buoyancy force is bigger than its weight.
buoyancy force
object
weight
If the weight is bigger than the buoyancy force, the object will sink.

4. 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.

5. Buoyancy in pictures Pressure increases with depth Low pressure at top
Pressure at the sides cancels because it is equal and opposite at all depths
Higher pressure at bottom

6. 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

7. 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

8. Buoyancy in pictures
Top force
Bottom
force
Buoyancy force