Topic 10 - Friction Drag

• Friction is a force that acts parallel to the interface of surfaces in contact and opposes their relative motion
• the coefficient of friction is a dimensionless scalar which is the ratio of friction quantity (between 2 bodies) and the normal reaction force.

  - the interaction between the molecules of the interfacing surfaces, the greater the size of the COF
• Coefficient of Static Friction: (resists the initiation of motion)

  • When a force is applied to attempt to move a stationary object over another surface. When sufficient force is applied to overcome static friction, the object will move
  • Will always be stronger than dynamic
  • more force is required to overcome static friction
• Coefficient of Dynamic Friction: (resists the motion of a moving object)

  • once object is in motion and 2 surfaces are sliding, friction is created
  • will always be smaller than static friction
• Maximising Frictional forces:

  • increase friction on the ground to enable direction changes e.g. footwear with grip (spikes, cleats, studs).
  • change playing surface e.g. astroturf, tarmac
  • use gloves to increase friction between hands/equipment e.g. gloves in baseball/golf
  • increase temperature/warming of surface to increase grip e.g. warming of F1 tyres
• Minimising frictional influences:

  • changing playing surface to make it smoother, reducing friction e.g. waxing floor
• Drag is the force or forces acting to oppose the motion of an object through a fluid medium such as air or water.
• Surface drag:

  • as a body moves through a fluid, its outer surface catches a layer of the fluid, slowing it down and causing drag.
  • can be minimised by changing the surface to reduce the interaction between surface and fluid.
  • e.g. using shark-skin suits in swimming to reduce friction between skin and water
• form drag:

  • as a body pushes against a fluid, the fluid pushes back (3rd law)
  • streamline the body and minimise the surface area facing the direction of motion to reduce form drag.
  • e.g. cyclists adopting a low-profile position to reduce drag
• wave drag:

  • when a body moves along the surface of a fluid some fluid is displaced in the form of a wave, these waves oppose motion
  • wave drag can be reduced by avoiding motion between air and water
  • e.g. swimming underwater for as long as allowed at the start of a race
• Factors influencing drag:

  Equipment for cycling (helmet and bicycle design)

  • Make modifications to the bicycle to make it more aerodynamic
  • Wear head gear e.g. swimming cap, cycling helmet
  • Swimming in a deck level filtering pool which is using specialised wave limiting lane ropes
• Factors influencing drag:

  Body position for a speed skater and swimmer

  • Use drafting by going behind another speed skater, cyclist or swimmer
  • Shaving down body hair
  • Swimming underwater as much as possible
  • Streamlining their body position