Friction is a contact force that opposes the relative motion (or attempted motion) between two surfaces. Air resistance is a specific type of friction that acts on objects moving through air.
How friction acts
Friction acts in the opposite direction to the motion or the direction the object would move. It converts kinetic energy into thermal energy in the surfaces. Without friction a moving object on a surface would never slow down.
Air resistance and terminal velocity
Air resistance increases as speed increases. When an object falls through air, weight pulls it down and air resistance pushes up. Initially weight is greater, so the object accelerates. As speed increases, air resistance grows until it equals weight. At that point the resultant force is zero, acceleration is zero, and the object falls at a constant speed called terminal velocity.
The sequence for a falling object:
- Object released: weight >> air resistance, object accelerates downward.
- Speed increases: air resistance increases, resultant force decreases, acceleration decreases.
- Air resistance equals weight: resultant force is zero, velocity is constant (terminal velocity).
Factors affecting air resistance
| Factor | Effect on air resistance |
|---|---|
| Speed | Higher speed increases drag |
| Cross-sectional area | Larger area increases drag |
| Shape | Streamlined shapes reduce drag |
Worked example
A skydiver of weight 700 N reaches terminal velocity. State the size and direction of air resistance at terminal velocity.
Air resistance = 700 N, acting upward. At terminal velocity the resultant force is zero, so air resistance equals weight.
Common errors and how to correct them
Stating that there is “no force” at terminal velocity. Both weight and air resistance still act. The resultant force is zero, not the individual forces.
Claiming air resistance is constant throughout the fall. Air resistance depends on speed; it increases as the falling object speeds up.
How to apply this in an exam
When describing terminal velocity, state explicitly that two forces become equal, the resultant force becomes zero, and therefore acceleration is zero. The examiner expects the cause (forces balanced), not just the effect (constant speed).
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