Air resistance and drag oppose motion. Change velocity, frontal cross-sectional area and surface, and watch the drag force respond. Double the speed and drag goes up four times.
Model: drag force = ½ × fluid density × drag coefficient × frontal area × velocity². You will not be asked for this equation in the exam, but it shows why velocity has the biggest effect. Surface and helmet effects are illustrative.
Pick an athlete, then try the Double the speed button.
The curve shows drag against velocity for the athlete you picked. It gets steeper as speed rises: twice the velocity gives four times the drag. A smaller frontal area or smoother surface flattens the whole curve.
A discus is shaped like an aerofoil. Increase its angle of attack and air travelling over the top goes further and faster than air underneath. Faster air means lower pressure, so a pressure gradient pushes the discus upwards: a lift force. Go too far and the airflow breaks away (stall).
Lift rising
Discus at 25 m/s. Lift rises with angle of attack up to a point, then the airflow separates from the upper surface: lift falls and drag rises sharply. The stall angle here comes from a simplified model, not a measured value. Lift doesn't need to beat weight: it reduces the net downward force, so the discus stays in the air longer.
Same rider, same 350 W of power, 1 km on the flat. The only thing you can change is how much air resistance they meet. Beat the target time in each round.
Predict first: which single change will save the most time?
Round 1: beat 81.5 s.
Quick-fire · Name the factor
AQA names four factors that affect air resistance and drag. Which one is each example mainly changing?
…
Pick an answer.
Factors that increase or reduce air resistance and drag:
Water vs air. Water is about 800 times denser than air, so a swimmer meets far more drag than a runner moving at the same speed. That is why swimmers work so hard on streamlining.
Downward lift force. Turn the aerofoil upside down, as with a racing car spoiler. Air underneath now travels further and faster, so pressure is lower below and higher above. The force acts downwards, increasing the normal reaction, so the maximum friction (grip) between the tyres and the track increases and the car can corner faster.
| Performer | What they do | Factor or force | Effect |
|---|---|---|---|
| Track cyclist | Low, tucked position | Frontal cross-sectional area | Less air resistance, higher speed for the same power |
| Track cyclist | Teardrop helmet, disc wheels | Streamlining and shape | Smoother airflow, less air resistance |
| Swimmer | Full-body suit, shaved body hair, swim cap | Surface characteristics | Less drag through the water |
| Downhill skier | Crouched tuck, arms in front | Frontal area and streamlining | Less air resistance at very high velocity |
| Discus thrower | Releases with an angle of attack | Lift force (Bernoulli) | Longer flight time, greater horizontal distance |
| Racing car | Inverted aerofoil spoiler | Downward lift force | Greater normal reaction, so more maximum friction (grip) and faster cornering |
Identify four factors that affect the air resistance acting on a cyclist. For each, explain how it affects air resistance and give one way the cyclist could reduce it.
A skier doubles her velocity from 15 m/s to 30 m/s without changing her body position. By what factor does the air resistance acting on her increase?
Draw and label a diagram of a discus in flight to show how a lift force is created.
Explain why a discus thrower should not release the discus with too large an angle of attack.
Explain how the spoiler on a racing car helps it go around corners faster.
Using the Bernoulli principle, explain how a discus thrower can increase the horizontal distance the discus travels.