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

6. Buoyancy in pictures Low pressure at top Pressures cause a force down on the top and a force up on the bottom Higher pressure at bottom

7. Buoyancy in pictures Small force down on top Pressures cause a force down on the top and a force up on the bottom Bigger force up on bottom

8. Buoyancy in pictures Top force Bottom force Buoyancy force