Drag Force & Air Resistance (CIE AS Physics)

• Drag forces are forces acting in the opposite direction to an object moving through a fluid (either gas or liquid)
• Examples of drag forces are friction and air resistance
• A key component of drag forces is that the magnitude of the drag force increases with the speed of the object
  • As an object speeds up, the drag force increases
  • As an object slows down, the drag force decreases
  • This will be explored further in Terminal Velocity
• Therefore, drag forces have the greatest effect at high speeds

• For a car traveling forward on a straight road:
• When the car is accelerating:
  • The driving force is greater than the frictional force
  • The resultant force is acting in the same direction as the direction of motion
  • Therefore, the car speeds up
• When the car is traveling at a constant velocity:
  • The driving force is equal to the frictional force
  • There is no resultant force acting on the car
  • Therefore, the motion of the car remains the same (continues traveling at a constant velocity)
• When the car is decelerating:
  • The driving force is less than the frictional force
  • The resultant force is acting in the direction that opposes the motion of the car
  • Therefore, the car slows down

Driving Force and Frictional Force Acting on a Car at Different Accelerations

For the car to accelerate, the driving force must be larger than the frictional force; for the car to travel at a constant velocity, the driving force and frictional force must be balanced; for the car to decelerate, the driving force must be less than the frictional force

• Air resistance is an example of a drag force
• Objects experience friction when moving through the air
• Air resistance depends on the shape of the object and the speed at which it is travelling
• Since drag force increases with speed, air resistance becomes important when objects move faster

Features of a Cyclist that Reduce Air Resistance

A racing cyclist adopts a more streamlined posture to reduce the effects of air resistance. The bicycle, clothing and helmet are designed to reduce air resistance to allow them to go as fast as possible