Drag Forces (OCR A Level Physics): Revision Note

Drag Forces

• Drag forces are forces that oppose the motion of an object moving through a fluid (gas or liquid)

• Examples of drag forces are friction and air resistance

• Drags forces:

  • Are always in the opposite direction to the motion of the object

  • Never speed an object up or start them moving

  • Slow down an object or keeps them moving at a constant speed

  • Convert kinetic energy into heat and sound

• Lift is an upwards force on an object moving through a fluid. It is perpendicular to the fluid flow

  • For example, as an aeroplane moves through the air, it pushes down on the air to change its direction

  • This causes an equal and opposite reaction upwards on the wings (lift) due to Newton's third law

Drag forces are always in the opposite direction to the thrust (direction of motion). Lift is always in the opposite direction to the weight

• A key component of drag forces is it increases with the speed of the object

• This is shown in the diagram below:

Frictional forces on a car increase with speed

Factors Affecting Drag

• The magnitude of the drag force depends on several factors, including:

  • The speed of the object

  • The object’s shape and texture

  • The density of the fluid

• The two most significant factors are the speed of the object and the object’s cross-sectional area

Air Resistance

• Air resistance increases with the speed of an object

• However, there are other factors that also affect the maximum speed, such as:

  • Cross-sectional area

  • Shape

  • Altitude

  • Temperature

  • Humidity

Many factors such as posture, clothes and bicycle shape must be considered when trying to reduce air resistance

Objects Falling in a Uniform g Field with Drag

• When an object is falling vertically in a uniform gravitational field, it is usually considered to have only a constant attractive force, weight, acting on it

  • However, for a more realistic approach, the opposing drag forces should be considered

• Opposing drag forces reduce the net force acting on an object and as a result, reduce the magnitude of the acceleration

  • The drag force will increase until it is equal in magnitude to the weight of the object

  • This leads to the object reaching a steady speed, known as its terminal velocity

• Because of drag, experimental values of the acceleration due to gravity, g, tend to be less than 9.81 m s^-2

Air Resistance & Projectile Motion

• Air resistance decreases the horizontal component of the velocity of a projectile

  • This means both its range and maximum height is decreased compared to no air resistance

A projectile with air resistance travels a smaller distance and has a lower maximum height than one without air resistance