David R. Hill

Mathematics Department

Temple University

Philadelphia, PA  19122


Lila F. Roberts

Math/CS Department

Georgia Southern University

Statesboro, GA  30460-8093





Demos with Positive Impact is an NSF project to develop a web-based collection of effective instructional demonstrations and to connect this resource to mathematics instructors.  This article provides an update on the project’s progress and showcases some of the demos that have been added within the last year. 


Demos with Positive Impact—An Ongoing Project


Demos with Positive Impact was founded in late 1999 as a project to develop a web-based collection of resources for teaching mathematics.  Funded by an NSF proof-of-concept grant, the focus is on instructional demonstrations that we or our colleagues have found to be useful and effective classroom tools.  The project has a broad scope, encompassing topics from secondary to undergraduate mathematics.  In contrast to projects or lab activities for students, demos are designed with the instructor in mind.  Thus, the project goal is to provide a collection of resources that the instructor can use to facilitate learning.


The demos within the collection are adaptable to a variety of learning styles and teaching environments.  Efforts are ongoing to provide versions of the demos for a variety of technology platforms, including platform-independent variations that run in a web browser.


To date, the Demos with Positive Impact web site has had visitors from countries such as Japan, China, Singapore, Australia, New Zealand, Czech Republic, Netherlands, Sweden, just to name a few.  Additionally, the audience has varied from middle and high school instructors to community colleges and four-year colleges and universities.  A sub-collection of the demos in the site has been featured in MathDL’s Digital Classroom Resources.  Recently, the site was awarded distinction by the Eisenhower National Clearinghouse as one of the Digital Dozen for October, 2002.


In the remainder of this article we showcase some of the recent additions to the Demos with Positive Impact collection and invite readers to contribute to the project as end-users and by providing feedback.


Enlightening Volumes:  Curve Fitting to Approximate Volumes

The purpose of this demo is to illustrate to students that techniques used to compute the volume of solids of revolution can be applied to real objects.  The demo employs digital photography and various curve-fitting techniques to approximate functions that, when revolved about an axis, yield a solid that approximates the object.  Here, the technique is applied to an ordinary light bulb and ideas for extensions to other areas are suggested. 


This demo is appropriate for calculus courses where volumes of solids of revolution are discussed.  It can be presented in courses, such as linear algebra and numerical analysis, where an emphasis on the curve-fitting aspect of the demo are appropriate.  Additionally, it can be extended to use in courses where numerical quadrature techniques are discussed.


The demo consists of two approaches.  In one approach, a photograph of a light bulb is imported into Geometer’s Sketchpad and points along the edge are selected.  The neck of the bulb appears much like a parabola.  Using Geometer’s Sketchpad, points are selected and a quadratic regression determines the quadratic function that approximates the data points.  In this demo, TI Interactive was used for the regression; alternatively a graphing calculator or computer software could be used. The round end of a bulb can be modeled using a circle.  Using Geometer’s Sketchpad to collect three points along the round end, a circle with form



was found to approximate the round end.  Figure 1 illustrates the bulb together with the curves that approximate the shape.  After the equations are determined, the volume of the bulb is approximated using standard integration techniques for solids of revolution (disk method).


An alternate approach is to use cubic splines to approximate the shape.  In this demo, the photograph was imported into MATLAB and points collected along the edge of the bulb formed the basis of a cubic spline approximation.  The action of the MATLAB routine (included with the demo) is illustrated in the animation in Figure 2.  The animation shows how the points are collected, the spline fit, and the resulting solid of revolution.


This is a versatile demo that can be incorporated into mathematics courses at several different levels using a wide variety of technology tools.  It is also a very nice example of a volume that would be difficult to directly measure and the power of the methods typically covered in calculus classes for computing volumes of solids of revolutions.  The demo also illustrates appropriate uses of the various available technologies.




Figure 1.  Parabola and Circle Fit to the Data.



Figure 4.  Cubic Spline Fit.  Click on the picture to go to the animation.


Visualizations for Volumes of Solids in Calculus


One of the most visited demos in the collection is actually a collection of demos that gives a comprehensive look at volumes of solids in calculus.  Solids are modeled using physical objects and the various techniques for approximating volumes are illustrated using animations (both animated gif and mov formats).  Galleries that contain examples typical of textbook problems provide a valuable tool for the study of volumes of solids in calculus.  Figure 3 gives examples of the collection.


Physical Example:  Sliced Bread

Volumes by Cross Sections

Click to see animation.

Physical Example:  Wedding Bell

Volumes by Disk Method

Click to see animation.

Physical Example:  Combo (the snack!)

Volume by Shell Method

Click to see animation.

Physical Example:  Angel Food Cake

Volume by Washer Method

Click to see animation.


Figure 3.  Volume Collection

The Resonant Filter


The purpose of this demo is to model and analyze the behavior of a resonant filter, including the solution of a second-order, constant coefficient differential equation, the notions of a transient and a steady state solution, and the idea of resonance.

The resonant filter is a circuit element containing a resistor with resistance R, a capacitor with capacitance C, an inductor with inductance L and a voltage source. Its response can be modeled by a second-order,  constant-coefficient, non-homogeneous second order differential equation 

, where q(t) is the charge on the capacitor at time t and Vin(t) is the (known) voltage from the input source.   The equation can be reforumlated in terms of current, i(t), using the relation that i'(t) = q(t).  Differentiating, we obtain an equivalent equation in terms of the current in the circuit,  





Let Vout(t) represent the voltage drop across the resistor.  The relationship between the voltage and current is given by Vout(t) = R i(t).  Thus, the second order differential equation equation, represented in terms of Vout is  




A program has been developed which takes as parameters the circuit elements, the source, and the initial conditions.  This program (included with the demo) provides an interactive utility for a numerical simulation of the effect on the output by varying the parameters and the input signal.  The program displays three components; the control panel, a graph of the input signal, and a graph of the output signal, represented by y(t). These are shown in Figure 4 for a particular input function.


This demo provides four activities.  A basic demo involves simply illustrating the solution to an initial value problem that models the resonant filter with one input signal.  Holding all parameters fixed except one, it is easy to investigate the effect on the output signal from changing the value of one of the physical parameters.  A second demo illustrates how solutions of a second order differential equation with constant coefficients and a sinusoidal forcing function decompose into two components—a transient solution and a steady state solution. Simply sliding the bars for the initial data shows quite clearly the presence of a transient solution that depends on the initial data, and a steady state solution that is unaffected by changes in the initial data. 

Two additional activities illustrate the importance of resonance in electrical devices.  Although resonance is often undesirable in many mechanical systems, such as a spring mass system, many electrical devices such as radios would not function properly without the phenomenon of resonance.  The program can be used to simulate a primitive radio.  This is a very interesting example of resonance in action which is relevant and real to students. 


Figure 4. Simulation of RLC filter with 
R = 0.52, L = 0.215, C = 0.215, y(0) = 1,
y'(0) = 0, and input signal



Constructing Equations from Word Problems


Our experience as mathematics instructors is that students often have a great deal of difficulty with the formulation of mathematical models for physical situations described in “word problems.”  The purpose of this demo is to provide a toolbox of visual aids for geometrically oriented word problems.  These visual tools are designed to help students to develop equations that provide an algebraic model for the problem.  These types of problems often form the basis for related rates and optimization problems in calculus so while this demo can be used in precalculus mathematics, it naturally leads to problem-solving at a higher academic level.

The demo provides a general outline of steps that apply to many verbal problems. The steps involved are illustrated with several examples. A collection of statements of geometrically oriented word problems is given together with visual demos (in the form of animations) that can be used within a lecture or assigned for students to use for practice.  Users are expected to supply the algebra to accompany the situation. The animations can also be used as 'preview' material for optimization problems.  Figure 5 gives a preview of illustrative examples.

General Problem Description

Animation Sample

a.  Cylinder formed by rolling a rectangle.

Click to see animation.

b.  Box formed by cutting out corners.

Click to see animation.

c.  Cone formed by cutting out a sector of a circle.

Click to see animation.

d. Cross-sectional area of an artery; fixed change.

Click to see animation.

Figure 5.  Examples from Word Problem Demo.

An Invitation to Participate


We invite you to participate in the Demos with Positive Impact project by sending us ideas that you have found to be successful for getting students’ attention and helping them to understand mathematical concepts.  We recognize that faculty members often are not encouraged nor rewarded for activities related solely to development of pedagogical tools.  While we cannot pay contributors for ideas, we do provide a mechanism by which mathematics instructors can get some recognition for innovative teaching strategies.  We invite you to visit the Demos with Positive Impact web site for an on-line opportunity to contribute to the project.  In addition, we welcome feedback on the collection or on individual demos within the collection.



Acknowledgements.  Partial support for this work was provided by the National Science Foundation’s Course, Curriculum and Laboratory Improvement Program under grant DUE-9952306.


Enlightening Volumes:  Curve Fitting to Approximate Volumes was contributed by Judy O’Neal, North Georgia College and State University, Dahlonega, GA.


Visualizations for Volumes in Calculus demo collection was contributed by David R. Hill, Temple University, Philadelphia, PA and Lila F. Roberts, Georgia Southern University, Statesboro, GA.


The Resonant Filter was contributed by Michael O’Leary, Towson University, Towson, MD.


Constructing Equations from Word Problems was contributed by David R. Hill, Temple University, Philadelphia, PA.