Wheels 1 Wheels. Wheels 2 Introductory Question The light turns green and you’re in a hurry. Will your car accelerate faster if you skid your wheels and.

Wheels 1

WheelsWheels

Wheels 2

Introductory QuestionIntroductory 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?

A.A. Skid your wheelsSkid your wheels

B.B. Barely avoid skiddingBarely avoid skidding

Wheels 3

Observations about Observations about WheelsWheels

Friction makes wheel-less objects Friction makes wheel-less objects skid to a stopskid to a stop

Friction wastes energyFriction wastes energy Wheels mitigate the effects of Wheels mitigate the effects of

frictionfriction Wheels can also propel vehiclesWheels can also propel vehicles

Wheels 4

4 Questions about 4 Questions about WheelsWheels

Why does a wagon need wheels?Why does a wagon need wheels? Why do sleds seem to “break free” Why do sleds seem to “break free”

and then slide easily when you shove and then slide easily when you shove them hard enough?them hard enough?

What happens to energy as a sled What happens to energy as a sled skids to rest?skids to rest?

How do wheels help a wagon coast?How do wheels help a wagon coast? What type of energy does a wheel What type of energy does a wheel

have?have?

Wheels 5

Question 1Question 1

Why does a wagon need wheels?Why does a wagon need wheels? Why do sleds work well only on snow Why do sleds work well only on snow

or ice?or ice?

Wheels 6

Frictional ForcesFrictional Forces

A frictional forceA frictional force opposes relative sliding motion of two surfacesopposes relative sliding motion of two surfaces points along the surfacespoints along the surfaces acts to bring the two surfaces to one velocityacts to bring the two surfaces to one velocity

Frictional forces always come in 3Frictional forces always come in 3rdrd law law pairs:pairs: Pavement’s frictional force pushes cart backwardPavement’s frictional force pushes cart backward Cart’s frictional force pushes pavement forwardCart’s frictional force pushes pavement forward

Wheels 7

The Two Types of The Two Types of FrictionFriction

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

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

limitlimit Sliding FrictionSliding Friction

Acts to stop objects that are Acts to stop objects that are alreadyalready slidingsliding

Forces have a fixed magnitudeForces have a fixed magnitude

Wheels 8

Question 2Question 2

Why do sleds seem to “break free” Why do sleds seem to “break free” and then slide easily when you shove and then slide easily when you shove them hard enough?them hard enough?

Wheels 9

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 10

Sleds and FrictionSleds and Friction

A stationary sledA stationary sled experiences static frictionexperiences static friction won’t start moving until you pull very hardwon’t start moving until you pull very hard

A moving sledA moving sled experiences sliding frictionexperiences sliding friction needs to be pulled or it will slow down and needs to be pulled or it will slow down and

stopstop experiences wear as it skids along the experiences wear as it skids along the

pavementpavement

Wheels 11

Introductory Question Introductory Question (revisited)(revisited)

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?

A.A. Skid your wheelsSkid your wheels

B.B. Barely avoid skiddingBarely avoid skidding

Wheels 12

Question 3Question 3

What happens to energy as a sled What happens to energy as a sled skids to rest?skids to rest?

Wheels 13

Friction, Energy, and Friction, Energy, and WearWear

Static friction (distance traveled is zero)Static friction (distance traveled is zero) No work and no wearNo work and no wear

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

A sliding box turns energy into thermal A sliding box turns energy into thermal energyenergy

Wheels 14

The Many Forms of The Many Forms of EnergyEnergy

GravitationalGravitational MagneticMagnetic ElectrochemicalElectrochemical NuclearNuclear

ElasticElastic ElectricElectric ChemicalChemical

Kinetic: energy of motionKinetic: energy of motion Potential: stored in forces between objectsPotential: stored in forces between objects

Thermal energy: the same forms of Thermal energy: the same forms of energy, but divided up into countless energy, but divided up into countless tiny fragmentstiny fragments

Wheels 15

Energy and OrderEnergy and Order A portion of energy can beA portion of energy can be

Organized – ordered energy (e.g. work)Organized – ordered energy (e.g. work) Fragmented – disordered energy (e.g. thermal Fragmented – disordered energy (e.g. thermal

energy)energy) Turning ordered energy into disordered energyTurning ordered energy into disordered energy

is easy to dois easy to do is statistically likelyis statistically likely

Turning disordered energy into ordered energyTurning disordered energy into ordered energy is hard to dois hard to do is statistically unlikely (it’s effectively impossible)is statistically unlikely (it’s effectively impossible)

Wheels 16

Question 4Question 4

How do wheels help a wagon coast?How do wheels help a wagon coast?

Wheels 17

RollersRollers

Eliminate Eliminate slidingsliding 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 18

WheelsWheels Eliminate Eliminate slidingsliding

friction at roadwayfriction at roadway Convenient because Convenient because

they don’t pop outthey don’t pop out Allow Allow staticstatic friction friction

to exert torques on to exert torques on wheels and forces wheels and forces on vehicleon vehicle

Wheel hubs still Wheel hubs still have have slidingsliding friction friction

Wheels 19

BearingsBearings

Eliminate Eliminate slidingsliding friction in wheel friction in wheel hubhub

Behave like Behave like automatically automatically recycling rollersrecycling rollers

Wheels 20

Question 5Question 5

What type of energy does a wheel What type of energy does a wheel have?have?

Wheels 21

Wheels and Kinetic Wheels and Kinetic EnergyEnergy

A moving wheel has kinetic energy:A moving wheel has kinetic energy:kinetic energy = ½ · mass · speedkinetic energy = ½ · mass · speed22

A spinning wheel has kinetic energy:A spinning wheel has kinetic energy:kinetic energy = ½ · rotational mass · kinetic energy = ½ · rotational mass ·

ang. speedang. speed22

Both kinetic energies are transferred Both kinetic energies are transferred via workvia work

Wheels 22

Summary about WheelsSummary about Wheels

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 frictionfriction

Powered wheels use static friction to Powered wheels use static friction to propel carpropel car