Simulating What You See

8/3/2019 Simulating What You See

1/7

MPTL16HSCI 2011Ljubljana 15-17 September 2011

SIMULATING WHAT YOU SEE: COMBINING COMPUTER MODELING WITH VIDEO

ANALYSIS

Douglas Brown, Cabrillo CollegeWolfgang Christian, Davidson College

AbstractThe Tracker video analysis and modeling program enables students to create particle model simulations

based on Newton's laws and to compare their behavior directly with that of real-world objects captured on

video. Tracker's "model builder" provides a gentle introduction to dynamic modeling by making it easy to

define and modify force expressions, parameter values and initial conditions while hiding the numerical

algorithm details. Because the model simulations synchronize with and draw themselves right on the video,

students can test their models experimentally by direct visual inspection, a process that is both intuitive and

discerning. A digital library browser enables users to open videos and models directly from web-based

collections such as the ComPADRE National Science Digital Library. Tracker is part of the Open Source

Physics project and is available for the Windows, Mac and Linux platforms from

or from the ComPADRE Open Source Physics collection at

.

IntroductionThe value of video analysis in physics education is well established (Beichner 1996, Laws 1998)

and both commercial and free educational video analysis programs are readily available (Tracker

2011, Logger Pro 2011, Coach 6 2011, VideoPoint 2011, Alberti's Window Motion Visualizer DV

2011). The video format is familiar to students, contains a wealth of spatial and temporal data, and

provides a bridge between direct observations and abstract representations of physical

phenomena. This has made video analysis attractive in many areas of physics including

mechanics, E&M, optics and even thermal physics (Brown 2009).

Interactive computer modeling has also been shown to be an effective learning tool (Christian2007, Jackson 2008). This approach actively engages students in the design, implementation and

analysis of mathematical models of physical phenomena. With simulation tools such as Easy Java

Simulations (EJS 2011, Esquembre 2004), students can easily vary model parameters and

expressions, visualize model behavior, and communicate results with others. The design and

analysis of computer simulations is in many ways similar to laboratory experimentation and often

leads the student to discover new insights into the behavior of a system.

This paper describes how computer modeling and video analysis are combined in Tracker, a free

video analysis and modeling tool produced by the Open Source Physics Project (Tracker 2011,

OSP 2011, Christian 2011, Brown 2007, Brown 2009) .

Video ModelingComputer modeling can be combined with video analysis in more than one way. For example,

model-generated data can be compared with video data graphically (Heck 2007, Heck 2010) or by

driving side-by-side animations (Kedzierska 2009). Both of these examples involve gathering video

data and then comparing the model with that data.

Tracker takes a different approach: the model simulations are drawn directly on the videos (Figure

1). Because the simulations and videos share the same time base and coordinate system,

students can test their models experimentally by direct visual inspection, a process that is both

intuitive and discerning. In effect, the videos make the models come alive while the models givethe videos a concrete object for comparison. Additional views of model-generated data such as


2/7

plots and tables are also available for analysis, and a tabbed page view enables authors to include

html documentation, instructions, or exercises.

Figure 1: Dynamic particle model of a cart bouncing on a tilted air track: video overlay

Tracker defines two basic types of particle models: (1) analytic and (2) dynamic. Dynamic particle

models in turn may be cartesian, polar or two-body systems that experience both internal and

external forces. All models are constructed using Tracker's "Model Builder" as shown in Figure 2.

The model builder provides controls for defining and varying parameters, initial conditions, and

position or force expressions. The expression parser used by the model builder accepts all

common mathematical functions. Unlimited undo/redo and instant visual feedback encourage

interactive exploration of models, and even small visual discrepancies can illustrate the limitationsof overly simplified models or video distortions.


3/7

Figure 2: Analytic particle model of a projectile: model builder

Analytic ModelsThe analytic particle model builder (Figure 2) has input fields x and y that allow students to specify

the position of the particle with time when the motion has a known analytic solution. When the

video is played, the particle is drawn on the video and its associated motion data are displayed in

the plot and table views. In addition to position and time, the user can choose to plot othervariables including velocity, acceleration, momentum and kinetic energy. Velocities and

accelerations are derived from position data using finite difference approximations.

Analytic modeling produces perfect data and helps students make the connection between the

motion of a particle and its mathematical description. Students learn to read graphs, estimate initial

conditions, and compare models with the real world. They also learn that analytic solutions can be

used in only a very limited number of situations.

Dynamic ModelsTracker provides a gentle introduction to dynamical modeling by making it easy to model a particle

that obeys Newton's laws. Students define and modify the force expressions, parameter valuesand initial conditions, and the model particle is automatically drawn on the video. At the same time,

user-selected motion data are displayed in the plot and table views. The motion is computed with

an ODE solver using a Runge-Kutta algorithm but the numerical details are hidden from students

so their learning focus is on the forces and resulting behavior.


4/7

Figure 3: Dynamic particle model of a falling coffee filter: model builder

Figure 3 shows the parameters and expressions used to model a coffee filter falling from rest in air.

Two forces are assumed to act on the filter: (1) a gravitational force defined by the parameters m

and g and force expression -m*g, and (2) a quadratic drag force defined by the parameter d and

force expression d*vy^2. This model can be shown to match well the motion of a falling cupcakecup with constant cross section even when its mass varies widely, unlike an alternative model that

assumes a linear viscous force (Brown 2009). Students are able to construct and test both models

convincingly without requiring analytic solutions, just as scientists do in current research.

Data AnalysisThe process of identifying pertinent forces, defining appropriate expressions and comparing

models visually by overlaying videos supports the modeling paradigm in a way that is both rigorous

and accessible even to students with no prior computational modeling experience and/or limited

data analysis skills. But since Tracker also provides traditional video analysis functionality,

students can easily analyze the model data with the OSP Data Tool, spreadsheet or other analysis

tool. The data tool makes it easy to compare model data with real-world data obtained from manual

or automatic tracking of video objects. In addition, since it is easy to clone and modify models,

students can visually and/or mathematically compare one model with another (Figure 4).


5/7

Figure 4: Data Tool comparing quadratic drag with linear viscous air resistance model.The area under the vy-t curve (gray) is the distance traveled between t = 0.1 and 1.1 s.

OSP Digital Library Browser

A complete Tracker experiment typically involves a video file, an XML document with extension

.trk and optional html, pdf and/or other documentation files. These files may be compressed into

a single zip file from which Tracker can automatically extract the experiment. The OSP Digital

Library (DL) Browser introduced in Tracker version 4.5 enables users to browse and accesscollections of such experiments located locally or on the web. Collections are stored as xml

documents that contain references to the actual resource files.

To open a collection file in the DL browser, the user may choose from a Collections menu or enter

a URL directly, just as with a web browser. Multiple collections may be opened in separate tabs.

Once open, any collection may be added to the Collections menu for easy future access.


6/7

Figure 5: OSP Digital Library Browser

An open collection is displayed in a tree as shown in the left pane of Figure 5. Each tree node is a

resource or sub-collection. Clicking a node shows a brief description of the resource, and double-

clicking the node opens the resource in Tracker.

One of the principle Tracker collections is in the ComPADRE National Science Digital Library.

ComPADRE offers the advantage that the authorship, modifications, and use of all resources are

documented and intellectually traceable. But the DL browser also enables users to build andorganize personal collections that can be uploaded to their own server. This gives teachers and

students a powerful mechanism for sharing videos and Tracker experiments.

Summary

Tracker is a free, open source video analysis and modeling tool that enables students to create

particle model simulations based on Newton's laws and to compare their behavior directly with that

of real-world objects captured on video. This offers the following advantages:

1. Students study real objects but focus on identifying forces rather than tracking and

analyzing motion.

2. Visual comparisons are rapid, intuitive and capable of discerning fine differences.

3. Visual agreement may validate models more convincingly than numerical or graphical

comparisons for beginning students.

4. Visual disagreement leads naturally to explorations of limitations and refinements of

models.

5. Quantitative analysis of model and/or video data can support and extend the visual

comparison.

In addition to the modeling capabilities described in this paper, Tracker can be used for traditional

video analysis. Features include manual and automatic point tracking, RGB line profiles, video


7/7

filters including perspective correction, and the integrated OSP Data Tool. Tracker is released

under the GNU GPL software license.

The OSP Digital Library Browser is unique because it enables teachers and students to organize,

share and access Tracker experiments and videos via web-based collections such as the

ComPADRE National Science Digital Library. ComPADRE has no registration costs because it is

part of the National Science Digital Library project and is endorsed and supported by theprofessional societies.

ReferencesBeichner R (1996) The Impact of Video Motion Analysis on Kinematic Graph Interpretation Skills, Am. J.

Phys. 64, 12721277

Laws P, Pfister H (1998) Using Digital Video Analysis in Introductory Mechanics Projects. Phys. Teach. 36,

282287

Tracker (2011) http://www.cabrillo.edu/~dbrown/tracker, accessed Aug 2011 (free)

Logger Pro (2011) http://www.vernier.com/products/software/lp, accessed Aug 2011 (commercial)

Coach 6 (2011) http://cma-science.nl/english/software/coach6/coach6.html, accessed Aug 2011

(commercial)

VideoPoint (2011) http://www.lsw.com/videopoint, accessed Aug 2011 (commercial)

Albertis Window Motion Visualizer DV (2011) http://www.albertiswindow.com/mv2d.htm, accessed Aug 2011

(commercial)

Brown D, Cox A (2009) Innovative Uses of Video Analysis, Phys. Teach. 47, 145150

Christian W, Esquembre F (2007) Modeling Physics with Easy Java Simulations, Phys. Teach. 45, 475480

Jackson J, Dukerich L, Hestenes D (2008) Modeling Instruction: An Effective Model for Science Education,

Sci. Educ. 17(1), 1017

EJS (2011) Easy Java Simulations, http://fem.um.es/Ejs, accessed Aug 2011

Esquembre F (2004) Easy Java Simulations: A Software Tool to Create Scientific Simulations in Java.

Comp. Phys. Comm. 156, 199204

OSP (2011) Open Source Physics, http://www.compadre.org/osp, accessed Aug 2011

Christian W, Brown D, Esquembre F (2011) OSP Tools and the ComPADRE OSP Collection. APS Forum on

Education Summer 2011 Newsletter, http://www.aps.org/units/fed/newsletters/summer2011/osp.cfm,

accessed Aug 2011

Brown D (2007) Combining Computational Physics with Video Analysis in Tracker. Proceedings, AAPT 2007

Summer Meeting, Greensboro, NC

Brown D (2009) Video Modeling with Tracker. Proceedings, AAPT 2009 Summer Meeting, Ann Arbor, MI

Heck A, Bruidegom B (2007) Bridging between Contexts and Concepts: How Data Video and Computer

Modelling Can Help. Electronic Proceedings of the IFIP Conference Informatics, Mathematics and ICT: A

'golden' triangle (IMICT 2007), Boston, MA

Heck A, Uylings P (2010) In a Hurry to Work with High-Speed Video at School. Phys. Teach. 48, 176181.

Kedzierska E, van Buuren O, Ellermeijer T, Uylings P (2009) AnimationsA New Coach Tool for Doing

Science. Proceedings, MPTL14, Udine

Simulating What You See

Documents

Transcript of Simulating What You See