Friction

Overview

  • An equal and opposite force created when an object is slid across a surface
  • Dependant on
    • the normal force (usually the mass of the object)
    • the co-efficient of friction (μ,"mu") between the surface and the object
  • μ is an experimental value, when comparing the μ of different materials, it is best to test yourself
    • The published μ for 2009 wheels was no found to be the actual value
  • Two different co-efficients of friction depending if the object is already moving (kinetic) or not (static)
    • You may have noticed when trying to slide a heavy object, it's really hard to get started, but once it starts moving, it's easy to continue moving

Forces

  • F = applied force
    • The amount of force used to try and move the object across the surface
  • Fg = gravitational force of object (Fg = m · g)
    • The mass of the object multiplied by gravitational acceleration
    • Fg = m · g
    • g = 9.81ms2
    • = 32.2fts2
  • FF = friction force
    • The amount of force the object and surface are resisting due to friction
  • FN = normal force
    • the amount of force the surface is providing to support the object

Example

  • General friction force equation
    • FF = μ · FN
  • In this example, Fg = FN
    • FF = μ · m · g
  • Static co-efficient of friction between wet concrete and rubber
    • μstatic ≅ 0.3
  • Assume rubber object weights 50 lbs
    • FF = μstatic · m · g
    • FF = 0.3 · 50 lbs = 15 lbs
    • FF = 0.3 · 22.7 kg · 9.81 ms2 = 66.8 N
  • In order to move this object, you must apply a force greater than 15 lbs or 66.8 N

Testing Friction

  • Testing static co-efficient of friction
  • Place object on surface of interest material
    • ie. place wheels on FRC carpet (or terrian for specific game)
  • Increase the angle of the surface until the object begins to slide
    • Ensure object cannot roll on surface
  • Record the angle (θ, "theta") at which the object begins to slide
    • μstatic = tanθ
  • When the object begins to slide
    • FN = Fg · sinθ
  • Recall the general friction equation
    • FF = μ · FN
  • In this example, FN = Fg · cosθ
    • μstatic = FgFg · sinθcosθ
  • μstatic = tanθ

Review of important equations

  • Relationship between linear and angular velocity
    • v = ω · π · D
  • Relationship between torque and force
    • τ = F · d
    • Relating angular velocity, torque, and tooth count
    • ω1ω2 = N2N1
    • τ1τ2 = N1N2
    • τ1τ2 = ω2ω1
  • Friction force equation
    • FF = μ · FN
  • Measuring static co-efficient of friction
    • μstatic = tanθ