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This 2 page Study Guide was uploaded by Usman Qureshi on Wednesday November 18, 2015. The Study Guide belongs to a course at a university taught by a professor in Fall. Since its upload, it has received 17 views.
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Date Created: 11/18/15
3 Last Lecture 3 Start of WorkEnergy 3Today 3 Blast from the past early exam review 3 More on WorkEnergy 3 Important Concepts 3 Work and Energy concepts can make some problems with complicated dynamics easier to solve 3 Think carefully abut how each force affects an object 3 Don t forget the angle factor and integral in Work 801 L Fall 2005 10252005 Important Reminders 3 Pset 6 due this Friday at 10am 3Next Mastering Physics deadline is next Monday 3Exam 2 is next Friday at 10 am 801 L Fall 2005 10252005 Work done by a Force 3 Not a vector quantity but vector concepts needed to calculate its value 3 Depends on both the direction of the force and the direction of the motion 3 Four ways of saying the same thing 3 Force times component of motion along the force 3 Distance times the component of force along the motion 3 WZFdcos6 where G is the angle between F and d 3 W J c where the s vector is along the path 801 L Fall 2005 10252005 Blast from the Past Early Exam Review 3Friction is not uN unless there are very special circumstances 3N is not Mg or Mgcos0 unless there are very special circumstances 3 ma is your friend trust what it tells you 3 understand the appeal of wanting to simply know and write down the answer but you need to resist 801 L Fall 2005 10252005 WorkEnergy Concepts 3Sometimes the effect of a given force on moving around energy is difficult to quantify or the effect is to move energy into or out of an object not considered part of the system 3n such cases it is convenient to use the work done by a force and say that work is change in energy 3ln this course energy will always consist of two components Kinetic Energy and Potential Energy 801 L Fall 2005 10252005 When to Use WorkEnergy 3So simple it never hurts to think about it first keeping in mind the limitations 3 Only one equation so only one unknown can be found 3 No information about time 3 No information about acceleration 3 No information about direction of velocities 3 Need information about all forces in the direction of motion of all objects 3 Standard problems involve speed at point A speed at point B and forces acting between A and B 801 L Fall 2005 10252005 Checklist to use WorkEnergy 3Clearly define what is inside your system 3Clearly define the initial and final conditions which include the location and speed of all objects 3Think carefully about all forces acting on all objects 3All forces must be considered in the Work term or in the Potential Energy term but never in both WzAEzEFinalE Initial KEFinal PEFinal Initial P EInitial 801 L Fall 2005 10252005 Potential Energy 3n general this is another form of real energy for example in Einstein s theory of gravity 3 For your purposes just think of it as another way to quantify the effect of a force on an object 30nly applies for a force with special properties 3Using Potential Energy instead of Work for these special forces greatly simplifies calculations 3 Especially helpful to get the signs correct 801 L Fall 2005 10252005 Calculating Potential Energy l 30nly the change in potential energy is important AU 2 APE PEat point B PEat point A APE 1361 A 801 L Fall 2005 10252005 Potential Energy 30nly applies for a force with special properties 3 Must be able to both take energy out from and put energy back into an object negative and positive work 3 Total work is zero if the endpoint and starting point of a motion are the same 3 For your purposes there are very few 3Gravity both near to and far from the Earth 3Springs 3Problems where you don t know the force but are given the Potential Energy explicitly 801 L Fall 2005 10252005 Calculating Potential Energy ll 3For simplicity the Potential Energy itself as opposed to the difference is always defined relative to some reference point where the potential energy is defined to be zero The location of this point is totally arbitrary because only the change in potential energy is ever important De ne arbitrarily PE at point A O B Then PEat point B JFd A 801 L Fall 2005 10252005 Checklist to use WorkEnergy 3Clearly define what is inside your system 3Clearly define the initial and final conditions which include the location and speed of all objects 3Think carefully about all forces acting on all objects 3All forces must be considered in the Work term or in the Potential Energy term but never in both WzAEzEFinalE Initial KEFinal PEFinal Initial P EInitial 801 L Fall 2005 10252005
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