ill LI I Lesson: How am) Supposed to Keep Track of It? (a 4aJ s (ri e t

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1 touching surfaces of two objects in a system warm). In lesson i, the work done on the system by the external force caused different types of changes in the system. Below, we describe each type of change in the system as a result of the work done on it. Devise a name for each type of change. You do work on a system to change its potential to do something (for example, to smash chalk orto make the 2.1 Describe H Lesson: How am) Supposed to Keep Track of It? 2010 Rutgers, The State university of New Jersey ill c) Positive work causes an increase in the elastic potential energy of the system. I - b) Positive work causes an increase in the kinetic energy of the system. f - a) Positive work causes an increase in the gravitational potential energy of the system. each experiment. Use materials on your lab table to show an experiment consistent with each item below, Identify your system for 2.2 Design an Experinient (With toys!) work? f) Describe the change of energy of a system if someone does positive work on it. What about negative answers to the traditional terms in the help box above. e) In parts (a) through (d), you came up with names for different types of energy. See if you can match your potential energy 1 and elastic potential energy. All of these fall under a larger category called mechanical energy. The change in the warmth of the surfaces of touching objects or their shape is called internal energy of the system. These changes are called energy changes. Each type of energy has a formal name: kinetic energy, gravitational Did You Know? R LI I d) The external force caused the surfaces of the touching objects to warm C c) Theexternal foice caused the slingshot to stretch. (ri e t b) The xternal force caused the cart to move faster and faster. a) The exterpal force caused the block to move higher above Earth s surface, j I (a 4aJ s Kreutter: Work and Energy

2 d) Kinetic energy in the system is converted to gravitational potential energy. 1* ) e) Kinetic energy in the system is converted to elastic potential energy. ) f3 Gravitational potential energy in the system is converted to internal energy. g) Gravitational potential energy in the system is converted to elastic potential energy. 23 Relate ç11 Describe one real-life situation that is consistent with the processes described below, Identify your system for each situation. a) Positive work causes an increase in the gravitational potential energy of the system. PU Ii i 2 cfv b) Positive work causes an increase in the kinetic energy of the system. c) Positive work causes an increase in the elastic potential energy of the system. d) Kinetic energy in the system is converted to gravitational potential energy. e) Kinetic energy in the system is converted to elastic potential energy. f) Gravitational potential energy in the system is converted to internal energy. / 2010 Rutgers, The State University of New Jersey

3 g) Gravitational potential energy in the system is converted to elastic potential energy. 2.4 Reason Examine the picture to the right. One of your classmates says, When the cargets to the edge it will have the ability tofall or falling ability. a) If Earth weren t there, would the car still have ability to fall? Explain. Nr t k1 iydtis prlem?11ow would b) ould we inclu& xcde carit 515 the analysis be different if we do? If we do not? (use the language of work and energy for your analysis). Did You Know? hq &t( LW J (((U, We have been examining a series of systems and analyzing the changes that occur to them. A system is an object or group of objects that we are interested in analyzing. REMEMBER! When we determine the objects in our system, we might need to include objects that aren t in direct contact, like Earth. 2.5 Observe and Reason Lift a box from the floor to a tabletop very, very slowly at a constant velocity. Assume that during this process you do a total of 125 J of work. (There are no changes in kinetic energy or internal energy ofthe system.) a) Identify the objects included in your system. What is not in your system? ) ectu b) Draw a picture of the initial and final states. Adapted from PM 201o Rutgers, The State university of New Jersey

4 c) Complete the table below. Portion of the Process Work that has been Gravitational Potential Energy of the done so far Box-Earth system Before you start, the box is on the floor. 0 You have lifted the box A of the way. You have lifted the box ½ of the way. You have lifted the box 3A of the way. - q You have lifted the box all the way to thetable, 125J I 2.6 Reason Go to the Physics Teaching Technology Resource website and investigate the experiment under Newton s Third Law (forces approach), where Eugenia and David push on each other: The above video depicts a situation where David and Eugenia are both on roller blades. First, David pushes Eugenia. Then Eugenia pushes David. a) Choose David as your system. b) Describe how the energy of the system changes after each process. c) Now choose Eugenia as your system. Describe how the energy of the system changes after each process. LW 1AC q ) d) Compare your results from David asyoursystem and Eugenia asyoursystem. What caused the change inenergyofeachsystern? - 1 tac e) What if you chose both David and Eugenia as your system? Where did the kinetic energy they both acquired come from? _J Rutgers, The State University of New Jersey

5 2.7 Observe and Reason Pull back the spring on a Nerf gun very, very slowly at a constant velocity. Assume that during this process there are no changes in kinetic energy or internal energy of the system. You then point the Neri gun vertically and release the trigger causing the dart to shoot into the air. The dart then comes back down and lands on the ground. a) Identify the objects included in your system. What is not in your system? Make sure to include all refrvant objects. e i fr rag. V S c,.e r v * 4 b) Assume you 215 J of work in total to compress the spring. Could you determine the work you do on the spring when the spring has been compressed half of the way? Why or why not? Consider if the force you exert on the spring is constant. ( / c) Complete the table below. Draw a picture of each portion of the process. What are your assumptions? Portion of the Process Picture Energy of the system Before you start, the Nerf gun is sitting in your hand im above the ground. I Ii You have pulled the spring back all the JW ) way. 5 You point the gun vertically and 4% release the trigger. 1 The dart flies up m in the air. I *-e 4 CL s_ The dart falls down and hits the floor 6Pt c where itstops. ICt1)Je4J4tI h (JOJ 28 Observe and Describe A system consists of a crate and a rough horizontal surface on which it sits( surface is made of a special material that changes color when it changes temperature. The rough You do positive work on the system by pulling the crate for about io m at a constant velocity. You observe the colors of the surface change indicating that the temperature increased. a) Draw a force diagram that explains why the crate is moving at constant velocity. A b) Describe how the system (crate and surface) is different after you do the work than before the crate ;. started moving 1f?tk tc ) i 2010 Rutgers, The State university of New Jersey

6 2olo Rutgers, The State University of New Jersey Homework ft,j Ok not in the system, how would you account for this change in energy of the box? e) You touch the bottom of the box and feel that it also increased in temperature. If the ground/surface was d) Revisit the choice ysm. Do you want!o make any changes? Write down your system below. 4 td 0 A c) If the ground/surface were not there, would the crate have warming potential? Should we include or Kreutter: Work and Energy c) Describe the types of energies and if there wa work done on yur system, b) Draw a picture of the initial and final states I a) You need to analyze energy transformation during this process. Identify the objects included in your and notices that the snow melts under her skis. Eva is learning how to ski and starts at rest at the top of a hill. While she is skiing down the hill, she gains speed How has the energy of the system changed? Explain your answer. speed of the puck by exerting a force on the puck-ice system over a distance of 4.0 m. He does -54.oJ of work. k f iii. The internal energy of the system increased. A hockey puck is sliding along the ice in an ice rink. Consider the puck and the ice as the system. Jon changes the Adapted from PIJM Jo Jc a (if 4 I4 V ct( system. What is not in your system? 2,10 Represent and Reason c J M I ii iv. The internal energy of the system decreased. Thekiitic energy of the system decre!d. kinetk energy of the system increased. 2.9 Reason tlec( 0t c cu ; ç C4 E flf ç 4P f14a (c0lmc CcL,taAArr \ 4 #f iej t4 t/ u iioc40jjtkp\ QjY exciudethegroundaspartofoursystem? Aio &rd.4ød 1 5 h1 44lC

7 Identify the objects a) Complete the table below Observe and Explain r: e(ft cu ( d) Describe any changes of energy of the system from the initial to final state. Kreutter: Work and Energy 201o Rutgers, The State University of New Jersey -_p b) Positive work on a system but with a value that is less than in part (a); d) Zero work on the system even though an object in the system moves. slides down on his snow sled. At V 1 the ground and places it on the. 7 Hector lifts a new television off Jeff starts at the top of a hill and k A f*- S), a) Positive work on a system; -r = moving really fast. Reflect: How is the physics concept of energy different from your use of the word outside of physics class? r OJ t) i; ii c) Negative work on a system;, tt bc) A)-i c PE t ) IA 4e C) 2i rv1 1 4 system s energy increases or decreases: Identify the system in each situation. Describe a real-life situation in which an external force does the following and state explicitly whether the 2.12 Reason, (, Earth ir. f - cj ) Co Aspring ikgblock the bottom of the hill, Jeff is k?i ia/ Qi2 TVstand. Television -p jj system. Describe the system. that are part of the Identify the initial and final state.