Drag Force

These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.

• Drag

  Drag is a force from a fluid that opposes an object’s motion relative to that fluid. Its dependence on speed, shape, and fluid properties varies with the physical regime. For example, a parachute increases air drag by presenting a large area to the moving air.

• Terminal speed

  Terminal speed is a constant speed reached when the forces along the direction of motion balance. For a falling object in fluid, increasing drag can eventually balance weight after buoyancy is also accounted for. For example, a falling raindrop can reach approximately constant downward velocity instead of accelerating indefinitely.