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Transcript
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?