Classical Mechanics Lecture 5

Today’sConcepts:a)FreeBodyDiagramsb)Forceduetostringsc)Forceduetospringsd)Forceduetogravity

MechanicsLecture5,Slide1

Stuff you asked about:

! thechangeindistancecomparedtothestandardlengthapplica6onsintheelas6cspringplease!

! Aproblemliketheelevatorproblem,butintwodimensionsoreven3dimensions.SomaybetheelevatorisfromWillyWonka'sfactory,andcanmovein3dimensions

! pleaserepeatandexplaintheprelectureslidewiththeelevatorexample...! showderiva6onofLawofUniversalGravita6onplease! I'dliketogetmoreofanexplana6ononforcetensioninsitua6onswith

objectslikehangingmassesonastring.! Whygravita6onalforceissosmallcomparedtotheothermajorforces?! Ididnotfindanyoftheseconceptsdifficultasformethiswasaquick

refresheronphysics11and12.

MechanicsLecture5,Slide2

! thechangeindistancecomparedtothestandardlengthapplica6onsintheelas6cspringplease!

! Aproblemliketheelevatorproblem,butintwodimensionsoreven3dimensions.SomaybetheelevatorisfromWillyWonka'sfactory,andcanmovein3dimensions

! pleaserepeatandexplaintheprelectureslidewiththeelevatorexample...! showderiva6onofLawofUniversalGravita6onplease! I'dliketogetmoreofanexplana6ononforcetensioninsitua6onswith

objectslikehangingmassesonastring.! Whygravita6onalforceissosmallcomparedtotheothermajorforces?! Ididnotfindanyoftheseconceptsdifficultasformethiswasaquick

refresheronphysics11and12.

Hurricane explanation

WeobservethathurricanesrotateCCWinthenorthernhemisphere,CWinsouthernhemisphere.

MechanicsLecture5,Slide3

Hurricane explanation

1)Ifyouareonsomeobjectrota6ngclockwisethenobjectsmovinginastraightlinerela6vetosomeonewhoisnotrota6ngappeartobeturningLEFTrela6vetoyou.

MechanicsLecture5,Slide4

2)Ifyouareonsomeobjectrota6ngcounterclockwisethenobjectsmovinginastraightlinerela6vetosomeonewhoisnotrota6ngappeartobeturningRIGHTrela6vetoyou.

Hurricane explanation3)ViewedfromabovetheNorthPole,theearthisrota7ngcounterclockwise.Thismeansifyouareinthenorthernhemisphere,thingswillalwaysappeartobeturningright.

MechanicsLecture5,Slide5

SUN

Hurricane explanation

4)Theatmosphericpressureatthecenterofahurricaneismuchlowerthantheatmosphericpressureintheairsurroundingit.Thismeansthatairtriestomovefromtheoutsidetotheinside.

MechanicsLecture5,Slide6

Hurricane explanation

5)Sincetheearthisrota6ng,alloftheselineswillturntotherightastheymovetowardthecenter.

MechanicsLecture5,Slide7

Hurricane explanation

6)Allofthese“rightturns”makethehurricanerotateCCW.

MechanicsLecture5,Slide8

MechanicsLecture5,Slide9

MechanicsLecture5,Slide10

MechanicsLecture5,Slide11

CheckPoint

MechanicsLecture5,Slide12

Aboxofmassmishungwithastringfromtheceilingofanelevatorthatisaccelera6ngupward.WhichofthefollowingbestdescribesthetensionTinthestring:

A)T < mgB)T = mgC)T > mg

a

CheckPoint

Aboxofmassmishungbyaspringfromtheceilingofanelevator.WhentheelevatorisatrestthelengthofthespringisL=1 m.Iftheelevatoracceleratesupwardthelengthofthespringwillbe:

A)L < 1 mB)L = 1 mC)L > 1 m

MechanicsLecture5,Slide14

m

aL

MechanicsLecture5,Slide16

I don't like elevators. They are either hot, smelly, and icky or really cold. They give you motion sickness if they go to fast, but if they don't they take forever. And they have to stop at like every other floor. They are usually always cramped and enclosed. You can die in there or get trapped in there, or both. If you move up, you get squished, but if you move down you fly. Talk about elevators please.

Clicker Question

YouaretravelingonanelevatoruptheSearstower.Asyounearthetopfloorandareslowingdown,youraccelera6on

A)isupward

B)isdownward

C)iszero

MechanicsLecture5,Slide17

Clicker Question

YouaretravelingonanelevatoruptheSearstower,andyouarestandingonabathroomscale.

Asyounearthetopfloorandareslowingdown,thescalereads

A)MorethanyourusualweightB)LessthanyourusualweightC)Yourusualweight

MechanicsLecture5,Slide18

CheckPoint

Ablocksitsatrestonahorizontalfric6onlesssurface.Whichofthefollowingsketchesmostcloselyresemblesthecorrectfreebodydiagramforallforcesac6ngontheblock?Eacharrowrepresentsaforce.

MechanicsLecture5,Slide19

Theonlyforcesac6ngontheboxaregravity(ac6ngdownward)andnormalforce(ac6ngupward).

Clicker Question

Ablockslidesdownafric6onlessinclinedplane.Whichofthefollowingsketchesmostcloselyresemblesthecorrectfreebodydiagramforallforcesac6ngontheblock?Eacharrowrepresentsaforce.

MechanicsLecture5,Slide20

A B C D

Clicker Question

Acartwithmassm2isconnectedtoamassm1usingastringthatpassesoverafric7onlesspulley,asshownbelow.Thecartisheldmo7onless.

MechanicsLecture5,Slide21

ThetensioninthestringisA)m1gB)m2gC)0

m2

m1

g

a

Clicker Question

Acartwithmassm2isconnectedtoamassm1usingastringthatpassesoverafric7onlesspulley,asshownbelow.Ini7ally,thecartisheldmo7onless,butisthenreleasedandstartstoaccelerate.

MechanicsLecture5,Slide22

Acerthecartisreleased,thetensioninthestringis

A)= m1gB)> m1gC)< m1g

m2

m1

g

a

Clicker Question

Ablockweighing4Nishungfromaropeaeachedtoascale.Thescaleisthenaeachedtoawallandreads4N.Whatwillthescalereadwhenitisinsteadaeachedtoanotherblockweighing4N?

MechanicsLecture5,Slide27

A)0N. B)4N. C)8N.

m m m

?