Buoyancy
Videos
Buoyancy, or upthrust, is a gravitational force, a net upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus, the pressure at the bottom of a column of fluid is greater than at the top of the column. Similarly, the pressure at the bottom of an object submerged in a fluid is greater than at the top of the object. The pressure difference results in a net upward force on the object. The magnitude of the force is proportional to the pressure difference, and is equivalent to the weight of the fluid that would otherwise occupy the submerged volume of the object, i.e. the displaced fluid.
A metallic coin (an old British pound coin) floats in mercury due to the buoyancy force upon it and appears to float higher because of the surface tension of the mercury.
A duck has difficulties to get under water due to its buoyancy. When no swimming forces are implied, the natural equilibrium of forces keeps about half of the duck off water.
Density column of liquids and solids: baby oil, rubbing alcohol (with red food colouring), vegetable oil, wax, water (with blue food colouring) and aluminium.
Convection
Videos
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity. When the cause of the convection is unspecified, convection due to the effects of thermal expansion and buoyancy can be assumed. Convection may also take place in soft solids or mixtures where particles can flow.
William Prout
Fireplace, with grate and chimney
This color schlieren image reveals thermal convection originating from heat conduction from a human hand (in silhouette) to the surrounding still atmosphere, initially by diffusion from the hand to the surrounding air, and subsequently also as advection as the heat causes the air to start to move upwards.