What term describes an effective force on fluid due to density or temperature differences in a gravitational field?

Buoyancy is the term that describes the effective force on a fluid arising from differences in density or temperature within a gravitational field. This phenomenon occurs because fluids tend to rise or sink based on their relative densities. For instance, when a substance is heated, it typically becomes less dense than its cooler surroundings, leading it to rise. Similarly, a denser fluid will sink in a lighter fluid. This behavior is fundamental in various scenarios, such as understanding how smoke rises during a fire or how hot air balloons operate.

In contrast, pressure refers to the force exerted by a fluid per unit area but does not necessarily relate directly to the effects of density differences. Velocity denotes the speed at which something moves, which is not specifically linked to buoyancy effects within a fluid. Viscosity measures a fluid's resistance to flow but does not address the buoyant forces that arise due to density and temperature disparities. Thus, buoyancy is the precise term that encapsulates the described phenomenon.