How are each of Newton’s Laws represented?

ReferenceFramesNewton’s1st LawistrueonlyforinertialreferenceframesIftheframeofreferenceisaccelerating,itisanon-inertialreferenceframeandNewton’s1stLawnolongerapplies

Goalsfortoday◦Learnhowtodrawafreebodydiagram,identifytheforcesactingonanobject,anddeterminewhethertheforcesarebalancedorunbalanced◦Understandwhenthemagnitudeofthenormalforceisgreaterthantheforceofgravity◦Understandtherelationshipbetweenfrictionforceandnormalforce

Whydoyoufeelheavierwhenyouareinanelevatorgoingup,andwhydoyoufeellighterwhentheelevatorisgoingdown?

Freebodydiagrams(FBD)(”ForceDiagrams”)RepresentobjectasapointDrawalltheforcesactingonanobject◦Thinkintermsofcomponents!Netforceisthevectorsumofalltheforces◦Gravity(FG)◦Directpushorpull(appliedforceFapp)◦Friction(fs orfk)◦Tension(FT):Strings,ropes,etc.◦Normalforce(FN)

Normalforce“Normal”means“perpendicular”Supportforce Table

“supports”book

FindingthenormalforceIfthesystemisinequilibrium–◦Normalforcekeepsequilibriuminthedirectionperpendiculartothesurface

Else:◦Usenetacceleration/netforcetofindnormalforce

Table“supports”book

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

Whyfrictionforcecan’tbemorethantheforceofapull

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

• Normalforceandfrictionforceareperpendiculartoeachother

• 𝜇s>𝜇k• Fs <𝜇sFN :Fs willmatchforceofthepullinordertokeepobjectinstaticequilibrium

• MaximumFsmax =𝜇sFN :thentheobject“slips”