Fluid drag and terminal speed
Terminal speed depends on the chosen drag model.
Key knowledge
For a specified body and regime, an approximate quadratic drag magnitude is F_D=ρC_DAv_rel²/2, opposing relative motion through the fluid. A vertical falling body reaches a terminal speed when gravity, drag and any buoyancy balance. Neglecting buoyancy in a constant-coefficient model gives v_T=√(2mg/(ρC_DA)).
Values and equations
F_D=½ρC_DAv_rel²; v_T=√(2mg/(ρC_DA))
- F_D
- Drag magnitude N
- ρ
- Fluid mass density kg m⁻³
- C_D
- Drag coefficient 1
- A
- Area projected perpendicular to relative flow m²
- v_rel,v_T
- Relative speed and terminal downward speed m s⁻¹
- m
- Body mass kg
- g
- Gravitational acceleration magnitude m s⁻²
Conditions and scope
- Positive fixed mass, fluid density, projected area and drag coefficient; still fluid and downward terminal motion in uniform gravity.
- C_D generally depends on shape and flow regime. This quadratic model does not replace the linear low-Reynolds-number drag law; the displayed v_T neglects buoyancy and assumes the regime remains applicable.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Read §6.4 drag equation6.5 and falling-body terminal-force balance6.8. Restore the explicitly scoped buoyancy neglect and regime dependence rather than using the illustrative coefficient as universal.
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Entry details
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- foundation-physics-drag-terminal-speed
- Revision
- foundations-refresh.1
- Structured reference record
- Foundational reference additions