Wheels 1

FrictionFriction

Wheels 2

Question:Question:

The light turns green and you’re in a The light turns green and you’re in a hurry. Will your car accelerate faster hurry. Will your car accelerate faster if you skid your wheels and “burn if you skid your wheels and “burn rubber” or if you just barely avoid rubber” or if you just barely avoid skidding your wheels?skidding your wheels?

Wheels 3

ObservationsObservationsAbout WheelsAbout Wheels

Without wheels, objects slide to a stopWithout wheels, objects slide to a stop Friction is responsible for this stoppingFriction is responsible for this stopping Friction seems to make energy disappearFriction seems to make energy disappear Wheels eliminate friction, or so it seemsWheels eliminate friction, or so it seems Wheels can also propel vehicles, but how?Wheels can also propel vehicles, but how?

Wheels 4

FrictionFriction

Opposes relative motion of two surfacesOpposes relative motion of two surfaces Acts to bring two surfaces to one velocityActs to bring two surfaces to one velocity Consists of a matched pair of forces:Consists of a matched pair of forces:

ObjObj11 pushes Obj pushes Obj22 while Obj while Obj22 pushes Obj pushes Obj11 Equal magnitudes, opposite directionsEqual magnitudes, opposite directions

Comes in two types: static and slidingComes in two types: static and sliding

Wheels 5

Types of FrictionTypes of Friction

Static FrictionStatic Friction Acts to prevent objects from starting to Acts to prevent objects from starting to

slideslide Forces can vary from zero to an upper Forces can vary from zero to an upper

limitlimit Sliding FrictionSliding Friction

Acts to stop objects that are already Acts to stop objects that are already slidingsliding

Forces have fixed magnitudesForces have fixed magnitudes

Wheels 6

Frictional ForcesFrictional Forces

Increase when you:Increase when you: push the surfaces more tightly togetherpush the surfaces more tightly together roughen the surfacesroughen the surfaces

Peak static force greater than sliding Peak static force greater than sliding forceforce Surface features can interpenetrate Surface features can interpenetrate

betterbetter Friction force drops when sliding beginsFriction force drops when sliding begins

Wheels 7

Question:Question:

The light turns green and you’re in a The light turns green and you’re in a hurry. Will your car accelerate faster hurry. Will your car accelerate faster if you skid your wheels and “burn if you skid your wheels and “burn rubber” or if you just barely avoid rubber” or if you just barely avoid skidding your wheels?skidding your wheels?

Wheels 8

Friction and WearFriction and Wear

Static friction (no distance)Static friction (no distance) No work and no wearNo work and no wear

Sliding friction (distance traveled)Sliding friction (distance traveled) Work and wearWork and wear Work is turned into thermal energyWork is turned into thermal energy

Wheels 9

RollersRollers

Eliminate sliding Eliminate sliding friction at roadwayfriction at roadway

Are inconvenient Are inconvenient because they keep because they keep popping out from popping out from under the objectunder the object

Wheels 10

WheelsWheels

Eliminate sliding Eliminate sliding friction at friction at roadwayroadway

Are convenient Are convenient because they because they don’t pop outdon’t pop out

Wheel hubs still Wheel hubs still have sliding have sliding frictionfriction

Wheels 11

BearingsBearings

Eliminate sliding Eliminate sliding friction in wheel friction in wheel hubhub

Behave like Behave like automatically automatically recycling rollersrecycling rollers

Wheels 12

SummarySummaryabout Frictionabout Friction

Sliding friction wastes energySliding friction wastes energy Wheels eliminate sliding frictionWheels eliminate sliding friction A vehicle with wheels coasts wellA vehicle with wheels coasts well

Free wheels are turned by static Free wheels are turned by static friction with the groundfriction with the ground

Powered wheels use static friction Powered wheels use static friction with the ground to propel the vehiclewith the ground to propel the vehicle