10 Friction and Drag

• Friction


  A force that acts parallel to the interface between two objects when they are in contact and opposes their relative motion
• Coefficient of friction


  A scalar quantity (dimensionless) that is a rate between the friction force and the normal reaction force
• Coefficient of friction


  • Dependent on the nature of the two surfaces and the material combinations
  • Dependent on the normal force (force with which they are pressed together)
  • Usually a value between 0 and 1, but it can be higher
• Static friction


  Friction that resists the onset of motion of two stationary surfaces
• Static friction


  • Can withstand motion up to a specific force (dependent on the coefficient of friction)
  • Once the force is exceeded, dynamic friction kicks in
• Dynamic friction


  Friction that resists the sliding of two moving surfaces past one another
• Dynamic friction


  • Occurs after the force overcomes static friction
  • Coefficient of dynamic friction is lower than the coefficient of static friction
  • Results in less friction and more motion
• Friction and sport


  • Shoes and surface (studs increase surface area and contact with playing surface, resulting in more friction and grip/stability)
  • Skiing/skating (flat and waxed skis or thin skates have less friction and more motion)
  • Gloves and handles (rougher glove material increases surface area and friction, resulting in better grip)
• Velodrome cycling


  • Thin tyre width to decrease surface area and dynamic friction
  • Very smooth surface to decrease friction
  • Brakes increase friction to decrease speed
• Factors affecting friction


  • Applied force
  • Texture of object surface (rough/smooth)
  • Nature/condition of playing surface (wet/dry, rough/smooth)
  • Age of surface (more worn down and rough)
  • Specialist equipment to manipulate friction (gloves, shoes, wax)
  • Mass of object (determines normal force)
  • Motion of objects (determines if it is static or dynamic friction)
  • Temperature (can affect friction as object is softer/harder)
• Drag


  The force acting to oppose the motion of an object through a fluid such as air or water
• Types of drag


  • Surface drag
  • Form drag
  • Wave drag
• Surface drag


  • Based on the interaction between the body surface and the fluid
• Form drag


  • Based on the shape of the object
• Wave drag


  • Generated when the body is at the surface of the fluid
• Factors influencing drag


  • Fluid viscosity
  • Surface size
  • Shape
  • Texture
  • Relative velocity
• Reducing drag in swimming

  • More time underwater
  • Streamline swim cap
  • Shaving hair
  • Head tucked in
• Reducing drag in cycling

  • Pointed arms
  • Lycra tight clothes
  • Crouched over position
  • Helmet shape
  • Narrow bicycle
  • Drafting (following closely behind another person)