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BouncingBall - STRIKING RESULTS WITH BOUNCING BALLS Andr...

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1 STRIKING RESULTS WITH BOUNCING BALLS André Heck, Ton Ellermeijer, Ewa K ę dzierska ABSTRACT In a laboratory activity students study the behaviour of a bouncing ball. With the help of a high-speed camera they can study the motion in detail. Computer modelling enables them to relate the measurement results to the theory. They experience that reality (measurements) is not automatically in line with the predictions of the theory (the models), but often even strikingly apart. This stimulates a process of repeated cycles from measurement to interpretations (how to adapt the model?), and in this way it realizes a rich and complete laboratory activity. The activity is made possible by the integrated ICT tools for measurements on videos made by a high-speed camera (via point-tracking), and for modelling and simulations. KEYWORDS Video recording and analysis, computer modelling, simulation, animation, kinematics, bouncing ball INTRODUCTION Each introductory physics textbook, already at secondary school level, illustrates Newton’s laws of motion and concepts of gravitational energy and kinetic energy with examples of objects dropped or thrown vertically and contains investigative activities about falling objects. The reasons are obvious: o the physics and mathematics is still simple enough to be accessible to most of the students; o an experiment with a falling object, in which data are collected with a stopwatch and meter stick, using sensors such as a microphone or a sonic ranger (MBL), or via web cams and video analysis tools (VBL), is easy to perform; o it is a clear invitation to compare measurements with theoretical results. Falling with air resistance is a natural extension of a free fall study. In this case, explorations are often directed towards two observable phenomena: (1) a falling object reaches a terminal velocity and (2) more massive objects fall faster than less massive objects. Experience from daily life already learns that air drag cannot be neglected in many practical cases, e.g., not for the motion of a table tennis ball. Im- portant factors that have an effect upon the amount of air drag are the speed and cross-sectional area of the falling object. In popular experiments of dropping coffee filters (Derby et al, 1997), balls and party balloons (Gluck, 2003), or paper cones (Wooning et al , 2006), students investigate the movement of an object released at a certain height and they determine the influence of weight, size and shape of the falling object on its motion. Data collection is typically done by stopwatch and meter stick, it is MBL- based using a sonic ranger (Gluck, 2003; Wooning et al , 2006), or it consists of video recording with a digital camera or web cam, followed by measurement in a video analysis system (Pagonis et al , 1997; Mooldijk and Savelsbergh, 2000; Cross, 2007). Computer modelling allows the students to compare reality (measurements) with theory-based prediction (the models). However, a practical investigation of various models of air resistance is often omitted because it is in practice not always easy to find
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