PHY111: Summer 201253

PHY111: Summer 201253 Lesson 02: Energy Part I Work-Energy Theorem Energy Transfer & Transformation 1/26

Applied Force INTRODUCTION to Work & Energy (“The Work/Energy Theorem”) 2/26

Applied Force Displacement INTRODUCTION to Work & Energy WORK is defined as the Applied Force times the Displacement W = F . d Work = 2 N * 3 m Work = 6 N.m Work = 6 Joules F = 2 N d = 3 m 3/26

Further thoughts on WORK If a Force is Applied, but no displacement or movement results, then NO Work is done on the Object. 4/26

Further thoughts on WORK The person might be expending ENERGY, but NO WORK is being done ON the object. 5/26

Discuss: What could have kept the box from moving? Think: If the box had never moved, how much work would have been done? W=Fxd=(4N)(0m)=0N.m=0J “Does this mean that no Energy was expended??” Answer: No, it simply means that even though Energy was expended or used, no work was done on the object. In order for work to be done on an object, there has to be a force applied and there has to be a displacement parallel to the direction of the force. 6/26

Further thoughts on WORK The person might be exerting a Force, but NO WORK is being done ON the object, since it is not moving 7/26

Further thoughts on WORK The person might be exerting a Force, but NO WORK is being done ON the object, since it is not moving We can just as easily replace the person with a metal pole, which exerts a force but expends no energy and does no work! 8/26

Further thoughts on WORK If the person is carrying the box horizontally, exerting a Force upward… 9/26

displacement Further thoughts on WORK No work is done on the box since the force is not in the direction of the motion… 10/26

displacement Further thoughts on WORK A force perpendicular to the displacement direction does no work on the object! 11/26

Further thoughts on WORK Even though the person is expending energy. 12/26

F = 10 N Work and Energy are Conserved If we use a Lever to move a Box upward… we exert a Force downward… The Load we are lifting is 40 N… 40 N 13/26

and work Output is 40 N x 0.5 m = 20 J F = 10 N So work Input is 10 N x 2.0 m = 20 J 2.0 m 40 N 0.5 m Work and Energy are Conserved The Box rises 0.5 m… The Force was applied through a distance of 2.0 m… 14/26

Work Input = 20 J Work Output = 20 J Work Input = Work Output Conservation of Energy (or Work) It is plain that the Energy to do Work is conserved: “You never get more Work out of something than the amount of energy you put in.” Think: What was the efficiency of this simple machine? … Is this possible? Why/Why not? … What was most likely the case, then? 15/26

Interactive Simulations • Below are some online simulations for you to explore that will help you gain a fuller understanding of Work and Energy concepts: [For the first two, just hit the Green “Run Now!” button, for the others hit “Play”]. • http://phet.colorado.edu/en/simulation/energy-skate-park • http://phet.colorado.edu/en/simulation/energy-skate-park-basics • http://physics.bu.edu/~duffy/classroom.html [“U” is potential energy] • http://physics.bu.edu/~duffy/classroom.html [again, “U” is potential energy, but in this case it is Gravitational PE instead of Elastic PE] 16/26

Energy Lab-Quest • I have put together an online lab-quest for you. Originally I made this with small lab groups in mind, but it is very easily done by individuals, as well. (The only difference is the level of discussion, for the most part). • On the following slides I will lead you through the assignment. This assignment should not take you more than the amount of time it would take to finish a class. However, since it is being given as an online assignment you may use as much time as you need (until Wednesday’s class, at which point you should turn this in for a grade). • Note that even though I gave you a link, these same files were uploaded to Blackboard yesterday (Sunday) for you. 17/26

Energy Lab-Quest (Intro.) • Print out (or organize a blank sheet for the responses) the following document: • http://mrlafazia.com/labs/quests/EnergyTransformations/EnergyTransformations.doc • For the text-files, I have written out mini-lessons (as if I were giving you a small “lecture”). You will then answer the “subquests” for each text-file. • For the video files, I have written questions on the “Energy Transformations” document, itself (as opposed to just leaving blank spaces). You may have to download the files first to view them or type out the URLs or download different codices or video players. • FIRST: Read Sections 5.7 – 5.12 in the text. (You have probably already read this. You may also want to supplement with online resources). 18/26

Energy Lab-Quest (Part 1/6) http://mrlafazia.com/labs/quests/EnergyTransformations/01.txt Read the mini-lesson from the above text-file and answer the Sub-Quest questions (#’s 1-5) 19/26

Energy Lab-Quest (Part 2/6) http://mrlafazia.com/labs/quests/EnergyTransformations/02.asx Watch the video-clip at the above link and answer the questions which are on the word document. 20/26

Grades/Assignments: Read Sections 5.13-5.16; 9.1-9.3. HW 1 should be turned in on Wednesday 25/26

Looking Ahead: Lesson 03 will take place in the Lab. We will go over some key Energy vocabulary, delve further into Energy Conservation, and carry out some related calculations. Lab 3 will also take place during this class. 26/26

PHY111: Summer 201253