Knowledge Organiser: Terminal Velocity
Key Terms
- Terminal velocity — constant speed when drag = weight
- Drag — resistive force opposing motion through a fluid
- Resultant force — net force after all forces added
- Weight — W = mg (downward gravitational force)
- Streamlining — shape designed to reduce drag
The Sequence to Terminal Velocity
- Weight > drag → resultant force down → acceleration
- Speed increases → drag increases
- Resultant force decreases → acceleration decreases
- Weight = drag → resultant = 0 → acceleration = 0
- Constant velocity = terminal velocity
V-T Graph Features
- Steep curve → decreasing curve → horizontal line
- Gradient = acceleration
- Area under graph = distance
- Parachute opening: steep negative gradient → new lower horizontal line
Factors Affecting Terminal Velocity
- Larger surface area → lower terminal velocity
- More streamlined → higher terminal velocity
- Greater weight/mass → higher terminal velocity
- Denser fluid → lower terminal velocity
Key Equations
- At terminal velocity: weight = drag force (resultant force = 0)
- F = ma (before terminal velocity, resultant force causes acceleration)
- Drag force ∝ v² (drag increases with the square of speed)
- W = mg (weight in Newtons from mass and gravitational field strength)
Common Mistakes
- Saying the object stops accelerating because forces disappear: At terminal velocity, forces are still acting — weight and drag are equal and opposite, giving zero resultant force, not zero force
- Thinking terminal velocity is the maximum possible speed: Terminal velocity depends on the specific conditions (shape, mass, fluid density) — a more streamlined object or greater mass gives a higher terminal velocity
- Confusing the velocity-time graph shape: The v-t graph for a falling object curves upward (decreasing gradient) before becoming horizontal at terminal velocity — the gradient (acceleration) decreases to zero, not the speed
- Saying drag increases linearly with speed: Drag force increases with the square of speed (∝ v²) — doubling speed quadruples drag, which is why terminal velocity is reached
- Forgetting the parachute phase on a skydiver graph: When a parachute opens, drag suddenly exceeds weight — the skydiver decelerates until a new (lower) terminal velocity is reached
Memory Aid
- More Weight → Wins the tug → terminal velocity goes UP
- More Drag (bigger area, less streamlined) → Damps the fall → terminal velocity goes DOWN