Definition and diagrams of Potential Energy

//ll   / l   /l   ’l  ’(l ). //ll i #  iilii ii l ’’s ’s  iill  iill 10272005 8.01L Fa2005 Last Lecture More WorkEnergy Start of PotentiaEnergy Today More on WorkEnergy including PotentiaEnergy Important Concepts Dont double count by including a force in both the Work and the PotentiaEnergy. Dont forget to include the Work term it is only zero under speciaconditions10272005 8.01L Fa2005 Important Remnders Pset 6 due tomorrow at 10am. Next Masterng Physcs deadne s next Monday. Exam #2 s next Frday at 10 am. Covers materiathrough todays lecture, tomorrowproblem-solving and Pset, and MondayMasteringPhysics assignment Experment #4 wbe done next Tuesday Info on exam and experment wbe posted soon //ll li/ Clliis “ii”  Clliinitial il itiiillill j(s)  illll ill j All iiin ial i. W=!E=EFinal"EInitial=(KEFinal+PEFinal)"(KEInitial+PEInitial)//ll ial  lliiial i(lli):  () ’l  () (3) 10272005 8.01L Fa2005 Checkst to use WorkEnergy eary defne what nsdeyour system. eary defne the and fnacondons, whch ncude the ocaton and speed of aobectThnk carefuy about aforces actng on aobects forces must be consdered n the Work term or the PotentEnergy term, but never n both10272005 8.01L Fa2005 PotentEnergy Ony appes for a force wth specpropertes so-caed conservatve forcesFor your purposes, there are very few: 1. Gravity both near to and far from the Earth2. Springs 3. Problems where you dont know the force but are given the PotentiaEnergy explicitly. You are only responsible for knowing the PE for gravity near Earth Mghand option on Exam #2. 1�//ll lliial  iliciial ilf () ililiiial iiliis iillilin ial ii Define (arbitrarily) PEat point A()=0Then: PEat point B()=!!Fid!sAB"//ll    GiFiiiial ll PEB()=!!Fid!sAB"Fx=!d(PE)dx Fy=!d(PE)dy ...10272005 8.01L Fa2005 Cacuatng PotentEnergy For smpty, the PotentEnergy tseas opposed to the differences aways defned reatve to some reference pont where the potentenergy s defned to be zero. The ocaton of thpont s totay arbtrary because ony the change potentenergy s ever mportant. 10272005 8.01L Fa2005 If: Then: ven , you can fnd PE and vce versa. Usng PotentEnergy to Cacuate Force //ll ial i iiti il illiil ii(li).  //ll ial i 10272005 8.01L Fa2005 PotentEnergy Dagrams PE as a functon of poson: If there are no other forces dong non-zero Work, then the totaenergy s a fat ne on ths pot. And the KE s the dfference see next sdeKE=ETot -PE 10272005 8.01L Fa2005 PotentEnergy Dagrams 2� PE (J) a b c d e x (m) ETot PE (J) x (m) ETot Small KE Large KE KE < 0 Unphysical!! KE = 0 Object stops Places where Fx = 0 Figure by MIT OCW. Figure by MIT OCW.