Work, Energy, and Power Name: Work-Energy Bar Charts Read from Lesson 2 of the Work, Energy and Power chapter at The Physics Classroom: http://www.physicsclassroom.com/Class/energy/u512c.html MOP Connection: Work and Energy: sublevel 6 The work-energy relationship is the most important relationship of the unit. The work done by external forces (Wext) is related to the total mechanical energy of the initial (TME;) and of the total energy of the final state (TMEf) of a system as follows: TME; + Wext = TMEf Your goal should be to combine your understanding of kinetic energy, potential energy, and work with the above equation in order to analyze physical situations involving energy changes and transformations and to solve computational problems involving work and energy. One tool that will assist in the analysis of physical situations is a work-energy bar chart. A work-energy bar chart represents the amount of energy present in a system by means of a vertical bar. The length of a bar is representative of the amount of energy present; a longer bar representing a greater amount of energy. According to the work-energy theorem, the initial mechanical energy (kinetic and potential) plus the work done on the system by external forces equals the final mechanical energy (kinetic and potential). Consequently, the sum of the bar heights for any initial condition must equal the sum of the bar heights for the final condition. Complete the following work-energy bar charts based on the given statement. Then cross out or cancel any terms in the work-energy equation that are either zero or the same on each side. 1. A ball falls from the top of a Initial Initial + Work = Final pillar to the ground below. The Energy Energy initial state is the ball at rest at KE PE the top of the pillar and the final state is the ball just prior to striking the ground. Ignore Fair. Final KEj + PEj + Wext = KEf + PEf 2 A car skids from a high speed to a stop with its Initial Energy + Work = Final brakes applied. The initial state is the car Energy traveling at a high speed and the final state is the KE PE KE PE car at rest. The force of friction does work on the car, thus changing the total mechanical energy. S + Initially Finally KEj + PEj + Wext = KEf + PEf The Physics Classroom, 2020 Page 1