Fluid mechanics: Downward lift force (speed skiers, cyclists, racing cars).

The idea

Lift is the component of the aerodynamic force acting perpendicular to the relative flow of air. For an aerofoil such as an aircraft wing, lift usually acts upward, but a body or surface can be shaped or angled so that the net perpendicular force points downward. This downward lift increases the normal contact force between the moving body and the surface, which raises the maximum available friction force. Speed skiers crouch low and keep their skis flat to generate downward lift that presses them onto the snow; cyclists adopt a low, forward position that pushes air downward onto the front wheel; racing cars use inverted wings and diffusers to create downforce that presses the tyres onto the track. In each case the downward lift improves grip and stability at speed, though it also increases drag and rolling resistance.

What earns the marks

• Lift is the component of the aerodynamic force perpendicular to the relative airflow.
• Lift can act downward when a body or surface is shaped or angled to deflect air upward.
• Downward lift increases the normal contact force between the body and the surface.
• A larger normal contact force increases the maximum friction force available for grip.
• Speed skiers, cyclists and racing cars use body position or inverted aerofoils to generate downward lift.
• Downward lift improves grip and stability but also tends to increase drag.

Common mistakes and how to put them right

• Thinking lift always acts upward.

  Put it right: Lift is perpendicular to the relative airflow and can point downward when the surface deflects air upward.

• Confusing downward lift with weight.

  Put it right: Downward lift is an aerodynamic force from the air, not the gravitational force on the body.

• Assuming downward lift reduces friction.

  Put it right: Increasing the normal contact force actually increases the maximum friction force.

• Believing downward lift has no cost.

  Put it right: Generating it usually increases drag, which can reduce top speed.

Exam tips

• Link downward lift to the normal contact force and then to friction, so the chain of reasoning is explicit.
• Use one named example, such as a racing car's inverted wing, and state the direction of the force on the car.
• Distinguish lift from weight and from drag when explaining the forces on a moving body.
• Check that any numerical comparison uses consistent units and that forces are given in newtons.