Dynamics: (b) factors affecting drag for an object travelling through air

The idea

For an object moving through air, the size of the drag force depends on several factors. Speed is the most important: drag increases as speed increases, and at higher speeds drag grows roughly with the square of speed. The cross-sectional area of the object also matters, because a larger area pushes more air aside. The shape of the object affects how smoothly air flows around it, so a streamlined shape reduces drag compared with a blunt shape. The density of the air is another factor, since denser air provides more resistance. Surface roughness can also influence drag by changing the flow near the surface. In calculations you may be told which factors are held constant so that you can isolate the effect of one variable.

What earns the marks

• Drag on an object in air increases as the object's speed increases.
• A larger cross-sectional area produces a larger drag force.
• A streamlined shape reduces drag compared with a blunt or flat shape.
• Air density affects drag: denser air gives greater drag for the same object and speed.
• Surface roughness or texture can affect drag by altering the airflow near the surface.

Common mistakes and how to put them right

• Assuming drag depends only on speed.

  Put it right by recalling that area, shape and air density also matter.

• Thinking a heavier object always has more drag.

  Put it right by noting that drag depends on shape, area and speed, not directly on mass.

• Believing a streamlined shape increases drag.

  Put it right by stating that streamlining reduces drag by allowing smoother airflow.

Exam tips

• When comparing two objects, identify which factors are controlled and which are changed.
• Use the term 'cross-sectional area' rather than just 'size' for precision.
• Link shape to airflow: streamlined shapes allow air to flow smoothly, reducing drag.