OCR A Level Physics

Revision Notes

3.3.5 Drag Forces

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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

Lift Force, downloadable AS & A Level Physics revision notes

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:

Drag force increases with speed, downloadable AS & A Level Physics revision notes

Frictional forces on a car increase with speed

Worked example

A car of mass 800 kg has a horizontal driving force of 3 kN acting on it.Its acceleration is 2.0 m s-2.What is the frictional force acting on the car?WE - frictional force on car question image, downloadable AS & A Level Physics revision notes

WE - frictional force on car answer image, downloadable AS & A Level Physics revision notes

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

Racing cyclist, downloadable AS & A Level Physics revision notes

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

Exam Tip

If a question considers air resistance to be ‘negligible’ this means in that question, air resistance is taken to be so small it will not make a difference to the motion of the body. You can take this to mean there are no drag forces acting on the body.

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

Projectile Motion Air Resistance, downloadable AS & A Level Physics revision notes

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

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