L29 - Friction & Traction

• Forces & Propulsion:

  • Consider, how we create motion
  • To propel yourself
  • Conditions required for us to propel our mass from one stride to the other?
  • Newton's 3rd law…
  • Need to apply a GRF in direction that opposes our desired motion & reaction forces causes propulsion proportional to the push we apply
  • But are there limiting factors to how big our push can be?
  • There are!
  • Causing propulsion vs sliding/slipping
  • Forces of gait for propulsion
• Friction:

  • The force that opposes the movement of 2 surfaces that are in contact with one another
  • Made up of 2 forms of force:
  • Force locking bw/ 2 smooth materials
  • Form locking material properties of traction
• Friction:

  • Made up of 2 forms of force:
  • ‘Force locking’ bw/ 2 smooth materials
  • Magnitude of force pushing 2 things together
  • Magnitude of up & down (the normal force [FN])
  • FN = mass x gravity (mg)
  • Is magnitude of force of force that is locking/connecting us to the ground/surface
• Friction:

  • Made up of 2 forms of force:
  • ‘Form locking’ material properties of traction
  • Mu (μ) = form locking
  • Describes material surfaces/properties
  • More rough the surfaces more likely there is interlocking
• Friction:

  • Fh = μs * FN
  • Fh = friction force
  • μs = surface traction
  • FN = normal force
  • Friction = resistance force that is independent of motion
  • Describes the amount of force required to cause these 2 contacting objects to slide across one another related to materials of 2 contacting surfaces
• Friction:

  • Fh = hz force required to move or slide an object related to mu & normal force
  • Acts independent of movement vector (nothing to do with movement vector)
  • What hz force got to apply to slide instead of propel/project → would have to be bigger than force of friction, larger than normal force applying, or something happening to surface (change surface)
• Friction - Sliding:

  • Friction force with external force, as increase external force
  • Can keep increasing hz force to a point will keep increasing & static direction will still allow us to propel ourselves
  • Once get to certain magnitude of external force it exceeds friction force (equation), then body starts sliding
  • Amount force required to keep us sliding is reduced
• Friction - Sliding:

  • Larger force required to initiate sliding
  • But once start sliding less force required to continue sliding
  • (less force than what was needed to initiate sliding)
• Friction - Sliding:

  • Static friction = up to point to initiate sliding
  • Kinetic friction/friction of motion = to continue sliding/after sliding is initiated
  • This graph tells us that Fs is always going to be greater than Fk
  • Fs > Fk
• Static or Sliding friction:

  • What does this mean?
  • Fs > Fk
  • Resistance force holding 2 things static (FN) & interlocking bw/ 2 surfaces (mu/μs)
  • Force of friction = to static friction (not moving, holding it in place
  • Doesn’t depend on movement, depends on 2 surfaces
  • Incline, magnitude acting in direction
  • Once slide no longer have to keep applying force for it to slide
• Sliding in Tennis… Novak Djokovic:

  • Utilising sliding as a tactic as it is quicker than running allows to change direction sooner
  • Hard surfaces designed for sliding to be difficult
  • So tricky tactic to master
  • Soft surface (top layer loose): create rolling friction making sliding friction easier (like wheels on a car)
• μs describes the traction dynamics ie shoe-surface interaction:

  • Describes how 2 surfaces interact
  • Traction describes interlocking of 2 surfaces = mu/μs
  • Friction = force bw/ 2 surface
  • Traction manipulate how 2 surfaces interact
• μs describes the traction dynamics ie shoe-surface interaction:

  • How shoes connect with surface
  • If wearing shoe not meant for surface greater risk for injury; greater forces on body than would expect
  • Testing mechanics image: how they test the surface (football shoes against artificial turf) - mimic normal forces of athlete, create rotation on foot model (torque), measure at what point does the shoe start to slide (limits of Fs (static friction))
  • Get mu (limit of static friction)
• μs describes the traction dynamics ie shoe-surface interaction:

  • Friction coefficient depends on type of shoe
  • Designed specifically for specific surface
  • To create traction forward & backwards (by creating interlocking)
  • Circle in basketball shoes bc/ they tend to pivot (helps maintain traction)
• μs describes the traction dynamics ie shoe-surface interaction:

  • Dependent on cleat depth & shape has different ability of shoe to slide
  • Low coefficient for skating bc/ want to slide
  • Harder ice lower coefficient easier to slide
  • Blade melts surface of water, turn dig into ice to get change in direction
  • Shape of blade changes - less blade in contact with ice to change direction for hockey players
  • If mu/μs greater or less than 1 - force needed to slide is significantly changed
• How does the Coefficient of friction can modify the Fh required to slide?

  • Fh = μs * FN
  • Rugby:
  • F μ > 1 then Fh must be very high to overcome the normal force
  • Hz force required to make them slide is going to be a lot greater, bc/ friction force, the μs (traction bw/ surfaces) is greater than 1
  • Whatever normal force take into account traction (μs), what's the limit of Fh for keeping the things still or creating sliding (above/scaling the normal force)
  • 1.35 * 800 N = 1080 N
• How does the Coefficient of friction can modify the Fh required to slide?

  • Fh = μs * FN
  • Ice Skating:
  • If μ < 1 then sliding may occur with Fh less than FN
  • 0.0003 * 800 N = 0.24 N
  • Force required significantly smaller to create sliding (for same mass)
• Normal Force & Friction:

  • The important thing to remember is that the force holding 2 objects together is always the normal reaction force
  • If the force is measured at an angle to the surfaces, you have to find out the magnitude of the normal component of it
  • Therefore it is easier to slide something on an incline than on the flat
• Normal Force & Friction:

  • Can alter angle to manipulate the friction force & sliding
  • Create an angle/incline → manipulate friction force required to create sliding (eg sled on snow)
  • Fs = μs (0.44) * FN (650 N) = 286 N
  • For static
  • Fs = μs (0.44) * FN (650 N) * cos(30°) = 247.7 N
  • with 30 degree incline
  • So force required to cause sliding is lower with incline
  • Incline affect amount of force required to create sliding
• So… We can alter the force required for sliding by altering the angle of the push…

  • Take a lunge, keeping body backward, keeping normal force directed at an angle
  • Keeps bulk of body back so that normal force has angle, this angle is creating a situation that will allow sliding to occur
  • Adjusting normal force by some angle
  • Fs = μs * (FN * cos(𝜽))
• Force required for a tackle

  • Ft = μ * Fn
  • Advantage if a player pushes up & forward to make a tackle
  • Why?
  • Have to overcome momentum + athletes interaction with ground (μs)
  • Alter direction of tackle to get friction out of equation (just deal with momentum)
• Force required for a tackle

  • Ft = μ * Fn
  • Advantage if a player pushes up & forward to make a tackle
  • In order to reduce friction component
  • Ball player: momentum + friction
  • Best way to counter that momentum is directly
  • However have friction to overcome as well; so tacking directly at player not most ideal (higher risk of injury to you & them due to that contact with ground)
  • Less dangerous if alter angle, with some upper lift force to offset normal force (reduce friction)
• Force required for a tackle:

  • Ft = μ * Fn
  • Advantage if a player pushes up & forward to make a tackle
  • Spreadsheet:
  • Tackler: player weight (800 N) & hz force (2000) & change angle
  • Look at how angle affect player with ball
  • To decide what is best upward angle
  • 10° decrease hz momentum by 31 N BUT friction (limiting ability to move player) is now 1810, offset mass of player enough to cause them to slide, now dealing with only momentum
• Friction & Injury:

  • Surfaces with greater translational & rotational resistance bw/ the shoe & surface have higher knee & ankle joint movements
  • Related to rotational injuries: ACL, ankle, ‘cleat catch’
  • ‘Cleat catch’: too long cleats, too much traction/stick
  • Instead of shoe, surface rotation/torque occurs at ankle or knee & can overcome resistance force of ligaments
  • Long term degeneration, tendinopathy, osteoarthritis
• Friction & Injury:

  • Artificial/synthetic surfaces may reduce sliding & rotational freedom; sliding allows reduction in loading by increasing the deceleration distance
  • Higher incidence of rotational injuries in 1st generation synthetic grass surfaces
  • 200% higher incidence of tennis injuries on non-sliding (synthetic) vs sliding (natural) surfaces
  • Increases with higher rotational injuries on artificial surfaces
  • μs (traction) normally higher
  • More varied reaction with temperature etc p can cause non-linear interaction
• Friction & Injury:

  • Traction component significant different to create a lot of torque on joints, that can increase risk of injury
• Sliding in Tennis is a risky technique…

  • Trying to slide, but bc/ shoe really sticks to surface (doesn’t slide) body keeps rolling over, rotational effect on ankle & knee (so rotate at joints, have to absorb that force)