The Physics of The Physics of SkateboardingSkateboarding

DesignDesign

Board: 7 plies of sugar maple woodBoard: 7 plies of sugar maple wood– Combined with polyvinyl gluesCombined with polyvinyl glues– Compressed at 300 psi (2.06 x 10Compressed at 300 psi (2.06 x 1066) for up to an hour) for up to an hour– Concave design – tails at about 20Concave design – tails at about 2000 angle angle– Covered in Grip TapeCovered in Grip Tape

WheelsWheels– Urethane: abrasion resistance, high traction and Urethane: abrasion resistance, high traction and

resiliency (restores shape after deformation and adds resiliency (restores shape after deformation and adds shock absorption)shock absorption)

– Turn about ball bearingsTurn about ball bearings

DesignDesign

Trucks:Trucks:– Axle, base plate, pivot Axle, base plate, pivot

pointpoint– Axle pivots on two Axle pivots on two

urethane cushionsurethane cushions– Allow wheels to swing Allow wheels to swing

in predefined arcin predefined arc

RidingRiding

Push off of the ground Push off of the ground with back footwith back foot

Turn by leaning left or Turn by leaning left or rightright

More common turn More common turn uses back wheel as uses back wheel as pivot for axis of pivot for axis of rotationrotation

Very Brief HistoryVery Brief History

Early Skateboarding:Early Skateboarding:– No one left the ground without rampsNo one left the ground without ramps– Power slidingPower sliding– ManualsManuals– Long BoardingLong Boarding

Rodney Mullen invents the Ollie Rodney Mullen invents the Ollie – Design changesDesign changes

The OllieThe Ollie

Rider begins with Rider begins with back foot on back finback foot on back finCrouches to allow Crouches to allow more room for vertical more room for vertical accelerationaccelerationPuts downward force Puts downward force on back fin as he on back fin as he jumps upjumps upFront of board pops Front of board pops upup

The OllieThe Ollie

Skateboard accelerates Skateboard accelerates vertically due to force of vertically due to force of the rider’s jumpthe rider’s jumpFront foot pulls the board Front foot pulls the board vertically using friction vertically using friction between the shoe and between the shoe and grip tapegrip tapeFront foot pushes front Front foot pushes front end down, back end rises end down, back end rises to meet back footto meet back footCreates illusion of the Creates illusion of the board being connected to board being connected to the rider’s feetthe rider’s feet

Spin (Rider with the Board)Spin (Rider with the Board)

Rider begins turn before jump, pushes off at an angleRider begins turn before jump, pushes off at an angle– Skateboard rises off at same angleSkateboard rises off at same angle

Or rider turns in mid air by rotating upper and lower body in opposite Or rider turns in mid air by rotating upper and lower body in opposite directions: small torque on arms translates to large one on legsdirections: small torque on arms translates to large one on legs– L = iw = p x rL = iw = p x r– i of arms > i of legsi of arms > i of legs– w of arms < w of legsw of arms < w of legs– Thus angular momentum is conservedThus angular momentum is conserved

The KickflipThe Kickflip

During ascent, rider kicks During ascent, rider kicks down on side of the board down on side of the board his foot is closest tohis foot is closest toCause a rotation about Cause a rotation about the axis from fin to fin the axis from fin to fin (long axis)(long axis)Spin occurs during Spin occurs during ascent, should meet ascent, should meet rider’s feet at peak height, rider’s feet at peak height, after already having after already having completed the spincompleted the spinHeelflip: Board is flipped Heelflip: Board is flipped at opposite end with at opposite end with outward kick of the heeloutward kick of the heel

Shove-it and Hard FlipShove-it and Hard Flip

Board rotates about axis perpendicular to Board rotates about axis perpendicular to the board, through the centerthe board, through the center

Riders pivots back wheels and then Riders pivots back wheels and then pushes down on the finpushes down on the fin

Hard to do while not moving, as front foot Hard to do while not moving, as front foot does not use friction to bring board updoes not use friction to bring board up

Hard flip rotates about axis through the Hard flip rotates about axis through the center, parallel to the boardcenter, parallel to the board

GrindingGrindingMoving on a surface without using the Moving on a surface without using the wheels – must overcome static frictionwheels – must overcome static frictionCan grind on Trucks or the bottom of Can grind on Trucks or the bottom of the deck (board slide)the deck (board slide)Rider contacts pole at peak of jumpRider contacts pole at peak of jumpTrucks have greater coefficient of Trucks have greater coefficient of kinetic friction than the board with most kinetic friction than the board with most surfacessurfacesFriction between two metals is less Friction between two metals is less than that between cement and metalthan that between cement and metal

Vert SkatingVert Skating

Performed on a half- Performed on a half- pipepipe– U-shaped, only ends U-shaped, only ends

are actually verticalare actually vertical– Sloped part is Sloped part is

transitiontransition

Increasing SpeedIncreasing Speed

Rider pumps on half pipeRider pumps on half pipe– Crouches down going into Crouches down going into

bottom of pipebottom of pipe– Raises up coming out of Raises up coming out of

bottombottom

Creates smaller radius Creates smaller radius about which the rider about which the rider rotates rotates – L= r x pL= r x p– Smaller radius means more Smaller radius means more

momentummomentum

Increasing SpeedIncreasing Speed

To go higher, rider must do To go higher, rider must do workworkRider works against centripetal Rider works against centripetal force to raise himself while force to raise himself while coming out of the pipecoming out of the pipe– He does work which leads to a He does work which leads to a

net energy gainnet energy gain

Larger ramps increase speed Larger ramps increase speed by increasing centripetal force by increasing centripetal force against which rider risesagainst which rider rises– More work translates to More work translates to

greater kinetic energygreater kinetic energy

Eventually, energy added with Eventually, energy added with each pump cannot overcome each pump cannot overcome energy lost to wind resistanceenergy lost to wind resistance

RecordsRecordsHighest flight off a half pipe: 23.5 ft = Highest flight off a half pipe: 23.5 ft = 7.16 m7.16 m– VV22 = Vo = Vo22 - 2a (distance traveled) - 2a (distance traveled)– Left Half pipe at 11.85 m/secLeft Half pipe at 11.85 m/sec

Longest jump: 75 ft = 22.87 mLongest jump: 75 ft = 22.87 m– Horizontal range equation, Horizontal range equation,

R=(VoR=(Vo22/g)sin2(/g)sin2(θθ))– He would have been going 14.97 m/secHe would have been going 14.97 m/sec

Both set by Danny Way at 2004 X-Both set by Danny Way at 2004 X-GamesGames– Ramp had 49 foot drop in and 20 foot Ramp had 49 foot drop in and 20 foot

quarter pipequarter pipe

RecordsRecords

The LoopThe Loop

Tony Hawk is the only Tony Hawk is the only Rider to have Rider to have completed a loopcompleted a loopSlowest speed Slowest speed needed to remain in needed to remain in contactcontacta = Va = V22/R, R = 5.49/R, R = 5.49-F(normal) – F(gravity) -F(normal) – F(gravity) = m(-a) = m(-V= m(-a) = m(-V22/R)/R)On verge of losing On verge of losing contact, F(normal) = 0contact, F(normal) = 0Thus -g = - VThus -g = - V22/R, and /R, and v = v = √(gR) = 7.334 m/s√(gR) = 7.334 m/s

FallingFalling

Mostly due to rider Mostly due to rider putting to great a putting to great a horizontal force on the horizontal force on the boardboard

Occurs frequently with Occurs frequently with grindinggrinding

ReferencesReferenceshttp://www.exploratorium.edu/skateboarding/http://www.exploratorium.edu/skateboarding/http://skateboarddirectory.com/articles/478516_the_physics_of_the_ollie.hthttp://skateboarddirectory.com/articles/478516_the_physics_of_the_ollie.htmlmlhttp://kingfish.coastal.edu/physics/projects/2000_Spring/skateboarding/http://kingfish.coastal.edu/physics/projects/2000_Spring/skateboarding/http://http://www.dannyway.comwww.dannyway.comHalliday et al. Halliday et al. Fundamentals of Physics 7Fundamentals of Physics 7thth Edition Edition. Wiley International . Wiley International Edition, 2005.Edition, 2005.