1)      

a)      In the absence of other forces, the net vertical force acting on a body is the difference between the weight of the body and the buoyancy force.

b)      Note that there is no gravity in space, and thus there can be no natural convection heat transfer in a spacecraft, even if the spacecraft is filled with atmospheric air.

3)      In heat transfer primary variable is temperature, and it is desirable to express the net buoyancy force in terms of temperature differences. This requires expressing the density difference in terms of a temperature difference, which requires a property that represents the variation of the density of a fluid with temperature at constant pressure. The property that provides that information is the volume expansion coefficient β

a)      The larger the temperature difference between the fluid adjacent to a hot (or cold) surface and the fluid away from it, the larger the density difference, the larger the buoyancy force and the stronger the natural convection currents, and thus the higher the heat transfer rate.

b)      In natural convection, no blowers are used, and therefore the flow rate cannot be controlled externally. The flow rate in this case is established by the dynamic balance of buoyancy and friction.