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Video phys

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  • 1. Video-Based Labs for Introductory Physics Courses Analyzing and Graphing Motion on Video Robert J. Beichner and David S. AbbottVideo-based labs (VBL) are a powerful Unfortunately, studies indicate that the axes, creating a graph of verticaltool for improving student understanding students do not share the vocabulary, position versus horizontal position (yof one of the most difficult and important nor are they able to easily comprehend vs. x) rather than a graph of velocitytopics in physics: graphs. This article de- graphs (Beichner 1994; McDermott, versus time (v vs. t). Unfortunately,scribes common student graphing difficul- Rosenquist, and van Zee 1987; van the “graph as picture” error can leadties, the history of VBL, techniques for to a correct position versus time graph. Zee and McDermott 1987). A valuableimproving student understanding of device for students facing these ob- This situation will occur whenever thegraphs, and software for acquiring andanalyzing video data. stacles is a video-based lab (VBL). The horizontal motion of the object is a link between graphing and kinematics linear function of time, making the makes video-based motion labs a par- graph as picture error difficult to de-K inematics, the study of motion, ticularly powerful tool for combating tect. Although this error is probably is critical to understanding some of the most common difficulties peculiar to kinematics, it indicates a physics. For that reason it is students encounter when they inter- general deficit in graphing skills,among the first topics covered in in- pret graphs. namely, students are not aware of thetroductory physics classes and it acts as The most frequently confronted abstract nature of graphing as a way toa foundation for much of the rest of problem when working with kinemat- represent data of a concrete situation.the course. ics graphs is the belief that a graph is Many students have trouble deter- Graphs are an important element in some form of photographic-like repli- mining slope. One critical misinterpre-the study of kinematics. Interpreting cation of the motion event. The tation is referred to as “slope/height”the large amounts of data displayed on “graph as picture” error might occurgraphs is critical to making sense of the when a student is asked to draw a ve-field. Because features on kinematics locity or acceleration versus time graph Robert J. Beichner is an associate pro-graphs, such as slopes and areas, are of a bicycle rolling down a hill and fessor of physics and David S. Abbottdirectly linked to observable physical over a small bump. The resulting is a National Science Foundationquantities such as distance traveled, drawing often duplicates the physical Graduate Fellow, physics department,speed, and acceleration, kinematics configuration of the given motion North Carolina State University, Ra-graphs are used as objects of study and event, right down to the bump at the leigh, NC 27695-8202; e-mail:a language of learning in physics in- end of the path. Beichner@NCSU.edu and DSAbbott@struction. In effect, the student misinterprets unity.NCSU.edu. November 1999 JCST 101
  • 2. confusion. Asked to indicate the point duce a graph of the object’s motion. video source are to be skipped betweenof maximum or minimum slope on a Videodiscs like Zollman’s Physics of captures.graph, students often pick the place Sports have proven to be very useful in The on-screen video controller iswith the largest or smallest ordinate helping students closely examine the used to advance to the beginning ofvalue, where the slope is actually zero motion of real objects. the desired sequence before clicking(Mokros and Tinker 1987; Barclay Computers have been used in phys- the record button. The video source is1986). Research by one of the authors ics labs to digitize movies and gener- automatically advanced through thefound other difficulties related to ate graphs of moving objects in the frames while the video capture pro-slopes, including students’ surprising video since the mid-1980s (Beichner gram directs the computer to captureinability to calculate the slope of lines 1989). The relatively primitive tech- images, compress them, and then storethat do not pass through (0,0) even nology available during those early ef- them on disk. Operation is the samethough they have little trouble finding forts resulted in very grainy black and whether students are collecting imagesthe slope of lines that pass through the white images. Objects in these early from a videodisc player or from a com-origin (Beichner 1994). digitized movies were difficult to rec- puter-controlled VCR—the lower Several tools are available to help ognize unless they were moving be- slider on the controller switches backstudents understand kinematics cause of the lack of color or even and forth between the two videographs. With the rapidly increasing shades of gray. sources.availability of technology inthe schools, many excellentinstructional packages havebeen developed. One of themost popular is Graphs andTracks—software that takesstudents back and forth be-tween a simulated ball roll-ing on a track and kinemat-ics graphs that describe theball’s motion (Beichner et.al. 1995). Ultrasonically based mo-tion detectors with accom-panying analysis software Figure 1. The control panel from a software package that lets students collect video from a videodisc player or VCR.packages have been thor-oughly studied and foundto be highly effective in motivating stu- Advances in computer technology Once the movie is saved to disk, itdents and helping them to acquire an now make the capture and playback of can be analyzed. About half a dozenintuitive understanding of kinematics high resolution movies quite easy. In different software packages are now(Brasell 1987; Redish, Saul, and fact, the hardware and software have available to do this (Gastineau 1995).Steinberg 1997; Thornton and advanced to the point where students The particular tool described here wasSokoloff 1998). More recently, soft- can concentrate on the physics de- created for use in introductory phys-ware packages for analyzing motion on picted in the videos and not on the ics laboratories at the high school andvideos have become widely available. techniques required to collect the data college level. It allows students to vid- The idea of using video to analyze (Beneson and Bauer 1993; Graney and eotape motion events and use themotion is not new. Early work by DiNoto 1995; Molnar 1995). graphing capabilities of a microcom-Dean Zollman, Robert Fuller, and The main control panel for a piece puter to carefully examine and analyzeothers involved the use of videodisc of software that automates the capture the motion. More specifically, theimages displayed on a television screen process is shown in figure 1. Students computer replays the video on its(Zollman and Fuller 1994). Students attach a computer-controllable VCR screen while simultaneously creating aplaced plastic sheets on the screen and or videodisc player to the video input graph of position or speed as a func-made a mark at the location of the ob- of their computer. They then specify tion of time.ject as it moved from frame to frame. how many frames will be “grabbed” Students can also “draw” motionThese marks were then used to pro- and how many frames of the original events using either painting programs102 JCST November 1999
  • 3. or a programming language. So, be-sides its obvious use as a data-gather-ing and analysis tool, video-based labsoftware can be used by students toanalyze previously recorded motionevents or even simulated microworldswhere the laws of motion are pro-grammed into the system by either thestudents or their teacher. We contend that by seeing both theconcrete motion event and the abstractgraphical representation of that mo-tion, students will be better able tomake the cognitive links between thetwo and may confront some of theirgraphing misconceptions (see Figure2). As the student steps through thevideo, the position of the object in thevideo and the corresponding point onthe graph are both highlighted. Toolsfor measuring slope and area from thegraph and distances and angles on thevideo can also be used to make criti-cal connections (see Figure 3). Figure 2. A screen from a video motion analysis program showing how the “graph as picture” misunder- Our research found instructional standing can be confronted. Students compare the graph of y vs. x (which mimics the actual motion) with abenefits for students at three different kinematics graph of y vs. t.high schools and a four-year college,suggesting that the VBL technique canbe useful in a variety of educational set-tings (Beichner 1996). We were par-ticularly pleased when our studiesshowed that females benefited as muchas males from the software. Video-based lab analysis can also beused to develop models for mechani-cal phenomena. Figure 4 shows resultsfrom using coffee filters to study theeffect of wind resistance on falling ob-jects. Because the filters can be stacked,the weight can be increased withoutchanging the cross-sectional area.When the filters are dropped, the stackspeeds up until the drag force equalsthe weight. After the terminal veloci-ties of stacks with different weights aremeasured, the data are entered into aspreadsheet and analyzed. The relationship between drag forceand velocity in this situation results ina graph of velocity squared versus the Figure 3. This combination of dialogs and windows from a video motion analysis program shows how stu-drag force, fitting a straight line dents can measure distances directly off the video and compare those data to the area under a velocity vs.through the origin. Data from a mi- time graph. November 1999 JCST 103
  • 4. References Barclay, W. 1986. Graphing misconceptions and pos- sibleremedies using microcomputer-based labs. Pa- per presented at the 1986 National Educational Computing Conference, University of San Diego, San Diego, CA. Beichner, R., M. DeMarco, D. Ettestad, and E.Terminal Speed (m/s) Gleason. 1989. VideoGraph: A new way to study kinematics. In Computers in Physics Instruction, eds. E. Redish and J. Risley, 244-245. Raleigh, NC: Addison-Wesley. Beichner, R., et al. 1995. Hardware and software preferences of high school teachers. Physics Teacher 33:270-274. Beichner, R. 1990. The effect of simultaneous mo- tion presentation and graph generation in a kine- matics lab. Journal of Research in Science Teaching 27:803-815. Beichner, R. 1994. Testing student interpretation of kinematics graphs. American Journal of Physics 62:750-762. Beichner, R. 1996. Impact of video motion analysis on kinematics graph interpretation skills. Ameri- can Journal of Physics 62:1272-1278. Benenson, W., and W. Bauer. 1993. Frame grabbing techniques in undergraduate physics education. Figure 4. Graph from a spreadsheet analysis of VBL and MBL data for falling coffee filters. Squaring both American Journal of Physics 61:848-851. axes gives a straight line indicating that the air drag force is proportional to V2. Brasell, H. 1987. The effect of real-time laboratory graphing on learning graphic representations of distance and velocity. Journal of Research in Sci- crocomputer-based (MBL) ultrasonic periments and the collection of data ence Teaching 24:385-395. Gastineau, J. 1995. Video analysis for Windows and position sensor are placed on the same from the students’ day-to-day Mac platforms. Physics Courseware Communicator graph for comparison to the VBL experiences. 3(1). results. It is important to realize that the Graney, C. M., and V. DiNoto. 1995. Digitized The flexibility in applying VBL purpose of VBL activities is not to video images as a tool in the physics lab. Physics Teacher 33:460-463. pedagogies is the key to improving stu- eliminate labs or replace them with McDermott, L., M.L. Rosenquist, and E. van Zee. dent motivation. As they use the same simulations. VBL provides students 1987. Student difficulties in connecting graphs tool in a variety of situations, students with a tool that can help in the study and physics: Examples from kinematics. American become very familiar with its operation of prerecorded real-world or artificially Journal of Physics 55:503-513. Mokros, J., and R. Tinker. 1987. The impact of mi- and evolve into “technology experts.” produced events. Students can analyze crocomputer-based labs on children’s ability to in- This has already been assessed in lab and real-world phenomena, either terpret graphs. Journal of Research in Science Teach- our earlier research where we found as homework or in class. VBL can also ing 24:369-383. that fully 80 percent of the students supplement other hands-on experi- Molnar, M. 1995. Creating video clips for instruc- tion. Physics Teacher 33:158-159. involved in the study would rather ences like MBL sonic ranger lab exer- Redish E., J. Saul, and R. Steinberg. 1997. On the use VBL than any other kinematics cises, give students an opportunity to effectiveness of active-engagement microcomputer- data collection technique (Beichner “play back” the motion to make sure based laboratories. American Journal of Physics 1990). they understand the critical aspects, 65:45-54. Thornton, R., and D. Sokoloff. 1998. Assessing stu- A major contributor to acceptance and help focus their attention on those dent learning of Newton’s laws: The force and of this instructional method is that stu- aspects of motion where known mo- motion conceptual evaluation and the evaluation dents are able to use the same data col- tion and graphing misconceptions of active learning laboratory and lecture curricula. American Journal of Physics 66:338-352. lection and analysis techniques for real- exist. van Zee, E., and L. McDermott. 1987. Investigation world situations as they do in school The versatile VBL approach is yet of student difficulties with graphical representa- lab studies. We believe that to get stu- another tool that teachers can utilize to tions in physics. Paper presented at the Second dents excited about science, it is vitally help their students learn the funda- International Seminar in Misconceptions and Educational Strategies in Science and Mathemat- important to help them see the science mental scientific skill of interpreting ics, Cornell University, Ithaca, NY. in their everyday world. VBL can be graphs and grasp one of the most Zollman, D., and R. Fuller. 1994. Teaching and specifically geared toward bridging the difficult and important topics in learning physics with interactive video. Physics To- gap between “artificial” laboratory ex- physics. t day 47(4): 41-47. 104 JCST November 1999

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