Whydoyoufeelheavierwhenyouareinanelevatorgoingup,andwhydoyoufeellighterwhentheelevatorisgoingdown?

Normalforceinanelevator10kgtoddlerinanelevator:

Elevatorhas0velocityand0accelerationSumFy =0,FN =FG =98.1N

Elevatoratconstantvelocity(0acceleration)SumFy =0,FN =FG =98.1N

2m/s2 accelerationSumFy = +20NFN=118.1N

-2m/s2 accelerationSumFy =-20NFN =78.1N

Freebodydiagrams(FBD)(”ForceDiagrams”)DrawalltheforcesactingonanobjectNetforceisthevectorsumofalltheforces◦Gravity(FG)◦Directpushorpull(appliedforceFapp)◦Friction(f)◦Tension(FT):Strings,ropes,etc.◦Normalforce(FN)

Normalforce“Normal”means“perpendicular”Supportforce

Findingthenormalforce◦Normalforceiswhatisnecessarytokeepequilibriuminthedirectionperpendiculartothesurface

Normalforceinanelevator10kgtoddlerinanelevator:

Elevatorhas0velocityand0accelerationSumFy =0,FN =FG =98.1N

Elevatoratconstantvelocity(0acceleration)SumFy =0,FN =FG =98.1N

2m/s2 accelerationSumFy = +20NFN=118.1N

-2m/s2 accelerationSumFy =-20NFN =78.1N

FrictionisaForce

Force on person by box

Force on floor by box Force on boxby floor

It’s the sum of all the forces that determines the acceleration.Every force has an equal & opposite partner.

Force on box by person

Force on floorby person

Force on personby floor

FrictionMechanismCorrugations in the surfaces grind when things slide.

Lubricants fill in the gaps and let things slide more easily.

StaticandSliding(Dynamic)Friction• Staticfrictionalforce(fs):inthedirectiontopreventsliding• Kinetic(sliding)frictionalforce(fk):oppositethedirectionofsliding• Staticfrictionalforcesalwaysgreaterthankineticones

Friction Force = Normal Force ´ (coefficient of friction)Ffriction = µ×Fnormal

• Normalforceandfrictionforceareperpendiculartoeachother• Coefficientofstaticfriction>coefficientofkineticfriction• Fs <𝜇sFN :Fs willmatchforceofthepullinordertokeepobjectinstaticequilibrium

• MaximumFsmax =𝜇sFN :thentheobject“slips”

Whyfrictionforcecan’tbemorethantheforceofapull

“Normal”ForcesandFrictionalForces

Weight of block

Decompose Vector

NormalForce

FrictionForce

Weight of block

ForceFrom Ramp “Normal” means

perpendicular

Friction Force = Normal Force ´ (coefficient of friction)Ffriction = µ×Fnormal

Janeispullinga10.0kgboxwithaforceof40.0Natanangleof30.0.Thecoefficientofkineticfrictionis0.1.A.DrawaFBDwithallforceslabeled.B.Determinethehorizontalacceleration(Hint:breakpullingforceintocomponents).Becareful:FN doesn’tequalFG!C.Howwouldthehorizontalaccelerationchangeifshepushedtheboxfrombehindwiththesameangleandforceinsteadofpulledit?D.Isiteasiertopushorpullanobject?

A.Whyisitmoredifficulttodoapull-upwith1armthan2arms?B.Ifyouweigh700N,Howmuchforceisin1armvs.2arms?◦1arm:700N,2arms:350NC.Ifbothofyourarmswereatanangleof30degreesfromthevertical,whatwouldtheforcebeineacharm?◦404N

TensionWhenaflexiblecordpullsonanobjectIfthecordismassless,thetensionwillbesamealongitsentirelengthIfpulleysareinvolved,assumetheyaremassless andfrictionlessRopesandcordscanonlypullForcealwaysactsalong(parallelto)theropeorcord

Bowserisusingpulleystoliftablocktothetopofhiscastle.Heisusingaropeloopedover2pulleys. Howmuchoftheblock’s2000Nweightdoeshehavetopullontherope?◦1000N

ProblemSolvingforForces1.DrawaFBDforeachobjectinvolved2.Identifytheforcesinbothdirectionsandmakeanetforceequation3.Iseitherdirectioninequilibrium?(Fnet =0)Aretherewayswecansimplifytheequation?4.Plugandchug!

Determinethecoefficientofkineticfrictionofapenguinslippingonflatseaweedifitsmassis21.8kgandforceduetofrictionis18.9N◦0.08

Mariaisslidingdownabannister.Ifshehasamassof45kgandthecoefficientofkineticfrictionbetweenMaria’sPJsandthebannisteris0.20,whatistheforceduetofrictionimpedinghermotiondownthebannister?◦76N