Factors that reduce and increase drag and their application to sporting…

The idea

Drag is influenced by velocity, cross-sectional area, fluid density and the drag coefficient. Increasing velocity increases drag because drag is proportional to v². Increasing cross-sectional area increases drag. Increasing fluid density (e.g., water vs air) increases drag. A higher drag coefficient (e.g., rough surface) increases drag. To reduce drag, athletes streamline their shape (reduce A), wear smooth clothing (reduce C_d), and reduce velocity if possible. For example, a swimmer wearing a smooth, tight suit reduces C_d and A, lowering drag. A cyclist crouching reduces A. Conversely, a skier in a bulky suit increases A and C_d, increasing drag. These factors apply to many sports: swimming, cycling, running, skiing and motorsport.

What earns the marks

• Drag increases with velocity squared, cross-sectional area, fluid density and drag coefficient.
• Drag can be reduced by decreasing cross-sectional area (streamlining), decreasing drag coefficient (smooth surfaces), or reducing velocity.
• Fluid density affects drag: water is denser than air, so drag is greater in water.
• Sporting applications include swimmers wearing smooth suits, cyclists crouching, and skiers wearing tight clothing.
• Increasing drag can be advantageous in some sports, such as using a parachute in parachuting or a drogue in sea kayaking.

Common mistakes and how to put them right

• Thinking that drag only depends on velocity.

  Put it right by listing all four factors.

• Believing that increasing surface area always increases drag.

  Put it right by specifying cross-sectional area.

• Assuming that drag is always undesirable.

  Put it right by giving an example where increased drag is helpful.

• Forgetting that drag is proportional to velocity squared.

  Put it right by stating the squared relationship.

Exam tips

• When asked to explain how to reduce drag, refer to specific factors: reduce cross-sectional area, use smooth materials, and reduce velocity.
• Use sporting examples to illustrate each factor, such as a swimmer wearing a cap to reduce drag coefficient.
• Remember that water is about 800 times denser than air, so drag in water is much greater.
• If asked to increase drag, think of parachutes, drogue devices, or clothing that increases area.