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Forces & Terminal VelocityOxford AQA IGCSE Physics: Revision notes

Section 1

What is drag?

An object moving through a fluid (a liquid or a gas) experiences drag — a resistive force that opposes its motion. Drag increases as the object's speed increases: the faster it moves, the greater the drag force acting against it.

Air resistance is drag caused by moving through air, and is a form of friction.

Key termsdrag

Section 2

How does a falling object reach terminal velocity?

When an object falls through a fluid (such as air), its motion goes through distinct stages:

  1. At the start, weight is the only significant force, so the object accelerates downward due to gravity
  2. As speed increases, drag increases too, reducing the resultant (net) force and so the acceleration
  3. Eventually, drag becomes equal in size to weight; the resultant force is zero
  4. From this point the object falls at a constant speed — its terminal velocity
Key termsterminal velocity
Common mistake

At terminal velocity the object is NOT stationary and NOT accelerating — it moves at constant speed because the resultant force is zero, not because there are no forces acting.

Section 3

How does a parachute change terminal velocity?

A parachute has a large surface area, which greatly increases drag for a given speed. This means the forces balance (drag = weight) at a much lower speed, so the parachute reduces the terminal velocity, allowing a safer landing.

Key termsparachute
Example

A skydiver falling freely reaches a terminal velocity of roughly 55 m/s; opening a parachute suddenly increases drag, decelerating them to a much lower terminal velocity for landing.

Section 4

How do we represent this motion on a velocity-time graph?

On a velocity–time graph for an object reaching terminal velocity:

  • The graph starts steep (large initial acceleration due to gravity, drag is small)
  • The gradient (acceleration) gradually decreases as speed increases and drag grows
  • The line becomes flat (horizontal) once terminal velocity is reached, showing constant speed and zero acceleration
Exam tip

When describing this graph, always link the changing gradient explicitly to the changing resultant force: steep gradient = large resultant force, flattening gradient = shrinking resultant force, flat line = zero resultant force.

Section 5

How does streamlining affect maximum velocity?

Streamlining shapes an object to reduce the drag it experiences at a given speed. With less drag, a greater speed is needed before drag balances the driving force (or weight), so streamlining increases an object's maximum/terminal velocity.

Key termsstreamlining

Must Know

  • Drag opposes motion through a fluid and increases with speed
  • A falling object accelerates until drag equals weight, then falls at constant terminal velocity
  • A parachute increases drag, reducing terminal velocity for a safer landing
  • On a velocity-time graph, gradient decreases as terminal velocity is approached, becoming flat (zero acceleration) at terminal velocity
  • Streamlining reduces drag and increases maximum velocity

That's the notes covered.

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