Potential / Kinetic Energy Remedial Exercise This Conceptual Physics exercise will help you in understanding the Law of Conservation of Energy, and its application to mechanical collisions. Exercise Roles: Manager Manages the group. Ensures that members are fulfilling their roles; the assigned tasks are being accomplished on time, and all members of the group are participating in the activities and understanding the concepts. Your instructor will respond to questions from the manager only (who must raise his or her hand to be recognized). Recorder Records the names and roles of the group members at the beginning of each day. Records the important aspects of group discussions, observations, insights, etc. The recorder ' s report is a log of the important concepts that the group has learned. Presenter or Spokesperson Presents oral reports to the class. These reports should be as concise as possible; the instructor will normally set a time limit. Reflector/ Strategy Analyst Observes and comments on group dynamics and behavior with respect the learning process. These observations should be made to the manager on a regular basis (no more than 20 minutes between reports) in an effort to constantly improve group performance. The reflector/ analyst may be called upon to report to the group (or the entire class) about how well the group is operating (or what needs improvement) and why.
Rubric Group Participation: 20% - Individual weighting for o How student performs in assigned role o How student contributes to success of group - Group interaction with class Exercise Completion: 40% - Completed exercise sheet - Correct responses Group - created Problems: 40% - Problems are creative, well defined - Solutions are correct, meet format requirements. Introduction Three topics are explored, and problems are practiced, related to Potential Energy, Kinetic Energy, and the Law of Conservation of Energy. Regarding Potential Energy, we will cover only Gravitational Potential Energy (GPE). In covering the Law of Conservation of Energy, I will restrict this investigation to the interconversion of gravitational potential energy and kinetic energy. Potential Energy: Potential energy, for this exercise, is the energy of position. Potential energy is energy stored in an object. This energy has the potential to do work. Gravity gives potential energy to an object. This potential energy is a result of gravity pulling downwards. The gravitational constant, g, is the acceleration of an object due to gravity. This acceleration is about 9.8 meters per second on earth. The formula for potential energy due to gravity is PE = mgh. As the object gets closer to the ground, its potential energy decreases while its kinetic energy increases. The difference in potential energy is equal to the difference in kinetic energy. After
one second, if the potential energy of an object fell ten units than its kinetic energy has risen ten units. Potential energy units are joules. Work is defined and quantified as the conversion of energy from one form to another. W = Fd in kg m/s2 meters Work equals force times distance a. John has an object suspended in the air. It has a mass of 50 kilograms and is 50 meters above the ground. How much work would the object do if it was dropped? Show your work. Work is converted in potential energy. m = 50 kg g = 9.8 m/sec 2 h = 50 m PE = mgh So, 50 9.8 50 24,500 SHOW YOUR WORK 1. Mrs. Jacobs dropped an object from 10 meters. She knows it did 50 joules of work. How much did it weigh? 2. If I raise a 2 kg book 0.8m over my head, determine the amount of work that I have done.
3. A bowler lifts her bowling ball a distance of 0.5 meters using 35 joules of energy. Her bowling ball has a mass of about kilograms. Kinetic Energy: Kinetic energy is the energy of motion. If an object is moving, it is capable of doing work. When you throw a ball, you do work on it to give it speed as it leaves your hand. The moving ball can then hit something and push it, doing work on what it hits. The equation that defines Kinetic Energy is : b. Determine the kinetic energy of a 625-kg roller coaster car that is moving with a speed of 18.3 m/s. 4. Missy Diwater, the former platform diver for the Ringling Brother's Circus, had a kinetic energy of 12 000 J just prior to hitting the bucket of water. If Missy's mass is 40 kg, then what is her speed?
5. A rolling wagon has 50 joules of kinetic energy. If the wagon s velocity is doubled, the kinetic energy of the wagon is: 6. The main difference between kinetic energy and potential energy is that A although both energies involve position, only potential involves motion. B kinetic energy involves position and potential energy involves motion. C kinetic energy involves motion and potential energy involves position. D although both energies involve motion, only kinetic involves position 7. Calculate the kinetic energy of a 45 g golf ball traveling at (a) 20 m/s (b) 40 m/s (c) 50 m/s 8. How fast must a 1000 kg car be moving to have a kinetic energy of (a) 2 10 (b) 2 10 9. Sir Galahad is attacking the castle of Dunkirk. His engineers design a battering ram to break down the front door of this formidable fortress. If the ram s mass is 1000kg and the maximum velocity it can attain is 5 m/s, what is the maximum kinetic energy that the ram can apply to the doors?
Conservation of Energy: Basically stated, energy cannot be created or destroyed, it can only be converted from one form to another. For the purpose of this exercise, we are investigating the conversion of gravitational potential energy (GPE) to kinetic energy, and back again to GPE. Here is the basic format of a problem that involves GPE / KE: c. A 3.00 kg toy falls off a shelf from a height of 10.0 m. Just before hitting the ground, what will be its kinetic energy? (Disregard air resistance. g = 9.81 m/s 2.) As the ball falls, its GPE completely converts into KE. Therefore, its KE just before it hits the ground, is virtually equal to its potential energy while it is sitting on the shelf. If the ball s PE is defined by the formula: Then PE = mgh PE = (3.00 kg)(9.81 m/s 2 )(10m) PE = 294. Joules, So, the ball s KE, which results from the total conversion of PE into KE, is 294. J
10. A hanging pendulum is released from rest at its highest point and allowed to swing freely, as indicated in the diagram. At which point would the pendulum bob have the greatest speed? Explain. 11. A 1 kg coconut falls out of a tree from a distance of 12 meters. Determine the coconut s kinetic energy the instant before it hits the ground. 12. Consider the diagram representing a portion of an amusement park ride. A 100 kg car has 30,000 J of potential energy at point A. Draw three pie charts including the percentages (or fractions) of energy for every point (you should have three charts).
13. Determine the potential, kinetic, and total energy at each point.
14. A 10 kg cart, starting from rest, sits atop a 100 m hill. How fast will it be moving when it reaches the bottom of the hill at point B? Create two examples of conservation of energy problems with your group. Provide the solutions, using the format described above. These problems may be used in the re-test of this unit. If your problem, or problems, will be selected, then your group will have a definite advantage in the re-test.