Which statement describes the relationship between particle density and settling rate in gravity settling?

• A. Higher density leads to slower settling
• B. Higher density leads to faster settling
• C. No relationship
• D. Only size matters

Answer: (B)

Gravity settling depends on the net downward force on the particle: weight pulling downward minus buoyancy and drag resisting motion. The weight scales with the particle’s density, while buoyancy depends on the fluid density. The key point is the density difference (rho_p − rho_f). A particle denser than the fluid has a larger net driving force, so it reaches a higher settling (terminal) velocity and settles faster. In the common laminar regime for small particles, the settling velocity is proportional to this density difference (and to the square of the particle diameter, with viscosity in the denominator). So increasing particle density relative to the fluid increases the settling rate. If the density matches the fluid, there’s no net force and little to no settling; if the particle is less dense, it tends to remain suspended or rise. Size also influences settling, but the density difference is the fundamental driver of faster settling when density increases.