Lab+7+-+Energy___updated

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University of North Carolina, Charlotte *

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202

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Mechanical Engineering

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Dec 6, 2023

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PHYS 2101 – Fall 2023 Lab 7 ENERGY Fig. 1: Investigating Energy Exchanges - Kinetic Energy and Gravitational Potential Energy Fall 2023

PHYS 2101 – Fall 2023 Objectives: At the end of this lab, students should be able to 1) calculate different forms of mechanical energy, and 2) verify the law of conservation of energy. 1. Introduction: The law of conservation of energy states that the total amount of energy in an isolated system remains constant. In other words, Energy can neither be created nor destroyed but can change its forms. The total energy E of a system (the sum of its mechanical energy and its internal energies, including thermal energy) can change only by the amounts of energy that are transferred to or from the system. If work W is done on the system, then W = ΔE = ΔE mech + ΔE th + ΔE int (7.1) If the system is isolated (i.e., W = 0): ΔE mech + ΔE th + ΔE int = 0 (7.2) The skate-park is an excellent example of the conservation of energy. For the isolated skate- track-Earth system, the law of conservation of energy equation has the form ΔE mech + ΔE th = 0 (7.3) The unit of Energy is Joules (J ) in the SI system. 1.1 Mechanical Energy : The mechanical energy E mec h of a system is the sum of its kinetic energy K and its potential energy, U; E mech = K + U (7.4) The conservation of mechanical energy can be written as; ΔE mech = ΔK + ΔU = 0. (7.5) It can also be rewritten as K 1 + U 1 = K 2 + U 2 (7.6) where the subscript refers to different instants during an energy transfer process. 1.2 Gravitational Potential Energy : The potential energy associated with a system consisting of Earth and a nearby particle is gravitational potential energy. If the particle moves from y 1 to height y 2 , the change in gravitational potential energy of the particle-Earth system is ΔU = mg (y 2 – y 1 ) = mgΔy. (7.7) 1.3 Kinetic Energy : The kinetic energy is associated with the state of motion of an object. If an object changes its speed from v 1 to v 2 , the change in kinetic energy is ΔK = K 2 – K 1 = ½ mv 2 2 - ½ mv 1 2 (7.8) Fall 2023

PHYS 2101 – Fall 2023 2. Procedure Please click here ( https://excelschools.net/en/simulation/energy-skate-park.html ) to download the PhET simulation link for Energy Skate Park . Take some time and play with the skater and his track . It helps to practice with the following features and controls. Track selector : click on ‘ Tracks’ located on the top right (Next to ‘File’) and select from the drop-down menu. For example, the “ Double well (Roller Coaster) ” shown above. Reset (located on the top right): This rests the simulation to default values and sets the track to friction parabola track. Skater selector : clicking on ‘ Choose skater…’ will allow you to choose a skateboarder with a different mass option. Measuring Tape: Check the ‘ Measuring Tape Bo x’ when you want to make measurements. Drag the left end of the tape measure to where you start your measurement and then drag the right end to the final location. To make a reference horizontal line to your measurement, check the potential energy reference box and drag the blue line you see on the screen to the initial position. Graph Selector: If you would like to observe graphs that depict the relationships among potential, kinetic, and thermal energy of the simulation, click the buttons under the Energy Graphs. The types of graphs are shown above. You can also add pie graphs by checking the show pie chart box. These graphs can be shown with or without Thermal energy . Gravity: you may change the gravitational force by changing the location or the sliding bar underneath the Gravity box. Additional Features : Clicking the C lear Heat makes the track frictionless. You can also edit the track friction and the skater mass using the Track friction and Edit Skater buttons. You can also control the speed of the skater using the slide bar under the screen. Whenever you are done playing make sure you reset the simulation and start the steps below: Part I: Parabolic Track 1. Click Reset and click on ‘Bar Graph’ , ‘Energy vs. Position’ , and ‘Energy vs. Time’ . Observe the energy bars as the skater moves back and forth. As the skater descends, his kinetic energy (green) ____________ and his potential energy (blue)_________. The ‘Total’ energy bar_____________. [3 Points] 2. Check the ‘ Measuring Tape box’ (please refer procedure section about how to use measuring tape). Considering the bottom of the parabola as a reference line, measure the maximum height ( h ) the skater climbs h = ________________ [1 Point] Fall 2023

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