Example Problems: Newton's Law

 ’  i i  Fil clill  ’ililiivil l ii il cliiiiiiLast Lecture Newtons Three Laws Today More discussion Lots of examples Important Concepts Think carefully about what object each force acts on Think carefully about which forces have well defined values and which one you need to calculate from F=ma Think carefully about the acceleration Important Remnders Pset # 3 due Frday at 10am. Exam #1 average = 64. naass grade wbe on a curve. We dont gve etter grade equvaents on ndduaexams. Generagudance s: >80 is good, <60 is not good, <45 is poor If you want to dscuss your exam or generaass performance, contact me or your rectaton nstructor durng offce hours or by appontment. ’1)iΣif Σ2) 3) jA jB is ally l iilliin diijB j !F=m!a!i iil!  iiill !F=m!a!ial  iipllllill“iial”  ilil jiis siit’iliiial ’lid  Alilii’it Newtons Three Laws If v s constant, then F must be zero and F=0, then v must be constant. Force due to obect on obect ways exactequan magntude and aways exacty opposte recton to the force due to obect on obect A. Some Advce Your nstncts are often wrong. Be carefus your frend. Trust what t tes you. InertReference Frames These prnces ony appy for observers who are not acceeratng, a so-caed nertframe There are phosophcaobectons to thconcept nce s hard to precsey defne an nertframe except as one where Newtons Laws are vaternatve formuatons are beyond the scope of ths course, you dont need to worry about 1()  i’|F|i“”  iiilicitly ilicitly “”  l ilicitly  l ll !F=m!a! !F=m!a! !F=m!a! llvil:(i)li ()  ()      … Fx!=max Fy!=may ...Forces you know about so farGravty near Earths surface: =Mg, ponts downStrng tenson: Can be gven expor mpor found from One example: String breaks if tension is larger than XX.Externaforces: Can be gven expor found from Normaforce: Amost aways found from Probem Song TooRevsedFree-Body Checkst Draw a clear diagram of eachobject Think carefully about all of the forces on eachobject Think carefully about the angles of the forces Chose an axis, put it on your drawing Think carefully about the acceleration and put what you know on your drawing Calculate components: Solvelili i l iii iitii  iiiiin lvilillialliiij illillliil l !F=m!a! !F=m!a! !F=m!a! !F=m!a!An Od Frend: Component Checkst Loop through vectors, s there a component? Is there an ange factor Is t sne or cosne? Is t posve or negatve? Summary Acton-reacton pars are an mportant concept song probems but need to be consdered very carefuy, especy the fact that the two forces n the par act on dfferent obects Thnk carefuy about whch forces can amost aways ony be determned usng Typicaexamples are the normaforce and string tension 2