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Transcript
VCE Physics: A SAC task for Unit 3 Outcome 3 Annotations of at least two practical activities from a practical logbook _____________________________________________________________________________________ Outcome 3 On completion of this unit the student should be able to investigate motion and related energy transformations experimentally, analyse motion using Newton’s laws of motion in one and two dimensions, and explain the motion of objects moving at very large speeds using Einstein’s theory of special relativity. To achieve this outcome the student will draw on key knowledge outlined in Area of Study 3 and the related key science skills on pages 11 and 12 of the study design. _____________________________________________________________________________________ Questions and considerations in running the pre-SAC experiments and the “in-class” SAC The following are some things for the teacher to consider for this type of SAC, and also possible suggestions (where relevant or applicable) have been made for the teacher’s consideration: Questions: Practical activities range from demonstrations, POE's, round robins of task, self-paced exercises, formal experiments to extended investigations. Should this assessment task for the Motion Area of Study focus on experiments? That form has been adopted for this document. What does 'annotation' mean? Is it commentary and answers to questions as the experiment is being conducted or is it done some time later? How many experiments should be the focus of this SAC? How should this SAC be implemented? (e.g. teacher provides a series of questions in relation to the experiments completed, with student responses forming the basis of the “annotations” required for this SAC) Is it acceptable to bring in some of the Key Knowledge bullet points from the Special Relativity part of this Outcome? Considerations - teacher could consider providing students with the objective(s), all materials/resources needed, and the procedures to be used for each experiment; this will ensure “base consistency” in how an experiment will be run, while also providing opportunities for students to offer suggestions to improve the experimental setup, as well as responding to questions that form the basis of this “annotations” SAC) - students can work independently and collaboratively during the course of the experiment, but the SAC is completed individually - teacher could allow students to choose the format for displaying their results rather than providing them with a table and headings, in order to allow the students to demonstrate their ability to record and display results using appropriate conventions (including units of measure) - by referring students to the procedure and results of their experiment in the SAC will give them a context for demonstrating an understanding of errors associated with each experiment, rather than simply recalling the key ideas relating to errors At Least Two Experiments Some possible experiments and questions which could form the basis of the annotations for this SAC are shown in the table below. Many other suitable experiments can be undertaken by teachers. An appendix at the bottom of this document provides some additional information. Experiments Questions that form a part of the Assessment? Investigating Newton’s 2nd Law on an Inclined Plane - students will demonstrate their understanding of forces in one and two dimensions - optionally students can explore the concept of work done as the cart moves up the incline Draw all the forces acting on the dynamics cart and the mass. What happened to the magnitude of the acceleration of the cart as the slope of the incline increased and everything else remained unchanged? Explain your answer with calculations. What happened to the magnitude of the acceleration of the cart if the hanging mass increased and everything else remained unchanged? Explain your answer with calculations. Identify two specific sources of equipment error that may have contributed to large percent differences between the theoretical and experimental methods for determining the acceleration of the system. Identify two specific sources of human error that may have occurred in this experiment. How would you modify this experiment to determine the effect that increasing the mass of the cart has on the motion given to it by a constant force? Projectile Motion (horizontal projection) - students investigate motion in two dimensions (including developing an understanding of the motion of a projectile in Earth’s gravitational field) - students will gain experience in determining the range of a projectile for different horizontal velocities - students will gain experience in calculating the initial horizontal for a given range and vertical displacement Write a definition for the term ‘projectile’. Discuss two methods for determining the horizontal velocity of the object in this investigation. How does the range compare to the horizontal velocity? Include a reference to your hypothesis (e.g. how did the relationship between range and velocity compare with your hypothesis?) What is the most important type of error in this experiment? Give an example that applies to this experiment. How would the launch velocity be different if the mass of the projectile was greater than that of the projectile used in the experiment? What about if the mass was less than the object mass used in this experiment? Explain, using a calculation what would happen to the range of an object, and its time of flight, if the projectile was launched with the same initial launch speed, but at an angle of 30º to the horizontal. Investigating Horizontal Circular Motion - students will be investigating the relationship between centripetal force acting on an object moving in a circle of fixed radius and the period of its motion - students will extend their understanding to solving circular motion problems on a larger scale Identify two physical quantities in this experiment that you attempted to maintain constant for each of your trials. Explain what is meant by the period of an object when it is undergoing circular motion. Provide an example calculation as part of your response to this question (e.g. a calculation to the problem “If an object revolves 50 times in 20 seconds, what is the period of the object?”). Identify the formula that determines the magnitude of the centripetal acceleration from the data that you are able to measure in this experiment (e.g. students should be able to measure radius ‘r’ of string and determine the tangential speed ‘v’). Sketch a graph of centripetal force as a function of the reciprocal velocity squared (i.e. graph of Fc vs 1/v2). Discuss the error that is introduced if the string is not moving in a horizontal plane (i.e. the stopper is allowed to sag). You should include specific reference to the stopper’s weight and its effect on the tension on the string. If the mass of the stopper were increased, describe what would happen to the force needed to keep the stopper in “orbit” if the tangential speed and radius remained the same? How would the orbital period change? Use calculations to support your answer. If the radius of the string was increased while maintaining a constant mass and tangential speed, how would the centripetal force change to compensate? Use calculations to support your answer. Based upon your understanding of acceleration in a horizontal plane, how would you expect that doubling the speed of the object to affect the centripetal force? The concepts examined in this practical task can be extended to satellite motion. Identify the cause of the centripetal force of a communications satellite which is in geosynchronous orbit about Earth. Note: question(s) requiring students to undertake calculations involving satellite motion could be potentially added. Energy Transformations (converting PE to KE) - using an inclined plane, students will roll a dynamics cart down the incline in order to investigate how the sum of the cart’s kinetic energy and the work done against friction compare with the cart’s loss of potential energy Explain the energy transformations that take place as the cart rolls down the incline (responses need to refer to the Law of Conservation of Energy?) Describe an experiment that you could perform to determine if the mass of the cart has any effect on the amount of energy required to overcome friction. If the cart was replaced by a block of wood on the inclined plane, how would you expect the values for - kinetic energy and work done against friction to change? What happens to the energy spent in overcoming friction? Conservation of Momentum - students will gain an understanding of momentum and the principle of conservation If both carts have the same mass, describe the velocity of each cart. When the connection between the two carts is ‘broken’, how do the forces exerted on each cart by each cart’s spring compare? How does the speed of the cart with greater mass compare to the cat with less mass? Explain what would happen to the momentum of the two carts if the springs were stiffer. Identify the two most likely sources of friction that could produce errors in this experiment. Discuss the consequences of the table (used to support your experimental setup) not being perfectly level. This experiment can be used to simulate a rifle firing a bullet. In the case of a rifle and the bullet, the bullet attains a high speed, but the rifle does not. Explain the reason for this. In the movie Eraser, an aluminium bullet is fired at near light speed. If the rest mass of the bullet is 250gram, then calculate the relativistic mass if the aluminium round was travelling at 0.8c. Changes in Potential Energy? students measure and graph the extension of a spring against force students drop of a mass from zero extension and measure the maximum extension. students use the Force vs extension graph and the dropped distance to generate energy versus distance graph for elastic potential energy, gravitational potential energy, then determine graphs for total energy and kinetic energy. Describe the force extension graph Use the graph to determine the spring constant Describe two ways to determine the spring potential energy stored in the spring Not all of the gravitational potential energy of the mass before it is dropped is transferred to spring potential energy at the bottom of the fall. Suggest where the missing energy may have gone. The graph of kinetic energy against distance can be used to determine the extension where the speed is greatest. Knowing the forces acting on the falling mass explain where you expect this point to be. Appendix The level of equipment and other materials will vary between schools. In theory, all of these practical activities can be run with basic equipment, or equipment which can be easily made, ideally by the students. Further information in this regards can be provided. Investigating Newton’s 2nd Law Possible Equipment/Materials - Dynamics cart - Pulley and string - Pulley clamp - Weights (various) - Ruler - Ticker timer, photogate, video analysis, Arduino microcontroller (or similar sort of data collection or recording equipment) Horizontal Projection Possible Equipment/Materials - large marble or steel ball bearing - tape measure or metre ruler - stop watch (or video analysis software & hardware) - carbon paper (or similar) to measure landing point of object Investigation Horizontal Circular Motion Possible Equipment/Materials - Balance (for measuring mass) - Glass tubing (or similar) - Masking tape or alligator clip (to fix the radius of rotation of the string) - Metal washers - Rubber stopper (or similar such mass) - Stopwatch - String (approx. 1 metre) - Paper clip to support washers Image credit: Taffel, Baumel & Landecker, 1966 Energy Transformations (converting PE to KE) g Conservation of Momentum Possible Equipment/Materials - two carts - spring system to provide simulated “explosion” - ticker timer, video analysis, Arduino microcontroller (or similar sort of data collection or recording equipment) http://cgscomwww.catlin.edu/sauerb/Ph12/PH12_Labs/PH12_Lab_17_Explosion_files/image002.jpg