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Physics1101W
(s)
Wednesday,February15
1
Theplan
• TodayandFriday:ContinueCh.4
• Monday:FinishCh.4,startCh.5
• Readingassignment:
• Today:4.1-4.4
• Friday:4.5-4.7
• Monday:5.1-5.2
• Don’tforget!Homeworkduetomorrow
• Don’tforget!Labreport#1dueFriday
Today’stopics
• Conceptreview:
• Circularmotion
• Newton’sLaws
• Newconcepts:
• Moredetailonhow/whyNewton’slawswork
(Review)ClickerQ:Uniformcircularmotion
DoestheInternationalSpaceStationuse
fueltostayinorbit?
(a)Yes (b)No
Chapter4:Forces
• Forcesarethecauseofall themotionwe’vestudied
sofar!
• Butifthereisno motionitdoesn’tnecessarilymeanthere
isno forceà sometimesforcescanbalanceeachother
out,andtheobjectstaysstill.
• Forceisavector,andmultipleforcescanadd
togetherusingvectoraddition.
• Thesumofallforces onanobjectdeterminesits
motion.
Examplesofforces
• Pushing andpulling
• Gravitational pulloftheEarth(weight)
• Electric andmagnetic forces(notcoveredin1101)
• Spring forces
• Tension forces
• Drag /airresistance
• Thrust (ex.thrustofarocketengine)
• Friction – forcefromrubbingagainstasurface
• Normalforce – theforceasurfaceexertsonan
objectincontactwithit.
IntroductiontoNewton’sLaws
Newton’sFirstLaw:Ifnoforcesactonan
object,itwillkeepdoingwhatitwasdoing.
• Ifatrest,itstaysatrest.
• Ifmoving,itdoesn’tchangevelocity.
Inotherwords,wecannothaveachangeinvelocity
withoutsomesortofappliedforce.
Example:rocketshipinspace
thatisnotfiringanythrusters.
Newton’sFirstLawofMotion
SandraBullockinthemovieGravity (2014)
• Anychangeinvelocityrequiresaforce.
• Changeinvelocityisacceleration.
àForceandaccelerationaredirectlyrelated.
Newton’sSecondLaw:
Forceequalsmasstimesacceleration
𝐹"#$ = 𝑚𝑎⃗ or
𝑎⃗ =
)*+,
-
• It’sthenetforcethatmatters.(Sumofallforces.)
• Forceisavector,sosumthemusingvectoraddition.
• Forceandaccelerationpointsameway.
NotesonNewton’sSecondLaw
𝐹"#$ = 𝑚𝑎⃗
• SIunitformassiskilograms
• SIunitforaccelerationism/s2
• Forcehasunitsof 𝑘𝑔
AB∗-
-
⁄>? =
AB∗≡
>?
𝑁
• 1Newton(N)is1 ? .
>
• Whataboutpounds?1pound(lb)=4.45N
Weightvsmass:
• Mass =howmuchmaterialisinanobject.
• Weight =howstronglytheEarthpullsonanobject.
• Massandweightarerelated!
• AslongasweareonEarth(!)thereisafixedrelationship
betweenmassandweightà peopleusekgtorefertoweight.
• Commonineverydaylanguage,butdon’tdoitinthisclass!
OnEarth,Maddy’s weightistwicethatofherlittle
sisterEmma.Thetwosistersgoonaspacemission
totheMoon.Whichofthefollowingisfalse?
A. Onthemoon,Maddy’s massistwiceEmma’s.
B. Onthemoon,Maddy’s weightistwiceEmma’s.
C. Onthemoon,Maddy’s freefallaccelerationis
twiceEmma’s.
D. Maddy’s massisthesameasonEarth,buther
weightandfreefallaccelerationchangeonthe
Moon.
Newton’sThirdLaw:
• Foreveryaction,thereisanequalandoppositereaction.
• Forcesalwaysactinaction/reactionpairs.
• Theforcesareoppositeandactondifferentobjects.
• Effects ontheobjectsaredifferentiftheyhavedifferent
masses:
Example:a10,000kgtruckcollideswitha1000kgcar.
Whichonesuffersmoredamage?
Checking Understanding
10-year-old Sarah stands on a skateboard. Her older brother
Jack starts pushing her backward and she starts speeding up.
The force of Jack on Sarah is
A. greater than the force of Sarah on Jack.
B. equal to than the force of Sarah on Jack.
C. less than the force of Sarah on Jack.
Slide 4-25
Forceshappeninaction/reactionpairs…butmostof
thetime,weonlywanttheeffectononeofthe
objects,notboth.
à Usually,we’llonlyconsideroneobjectatatime.
Example:Reactioncarts
Summary:Newton’slaws
1. Ifnoforcesactonanobject,itwillkeepdoingwhat
itwasdoing.
2. Forceisequaltomasstimesacceleration:
𝐹"#$ = 𝑚𝑎⃗
3. Foreveryaction,thereisanequalandopposite
reaction.
Drawingforcevectorsandfreebody
diagrams.
•
•
•
•
Drawapointfortheobject.
Drawthecoordinatesystem.
Draweachforcevector.
Drawthenetforcevector.
Afewreminders:
• Forceisavector.
• Todeterminemotion,it’sthenetforce thatmatters.
ConceptualquestionsfromCh 4
Threeforcesactonanobject.Inwhichdirectiondoes
theobjectaccelerate?