The Equilibrium and Elasticity section of the Physics_1 online course published by M. Casco Associates includes some mathematical models of real objects so that the reader may experiment with the ideas presented in the text. The pendulum disk model is displayed in the framework shown at the right. The pendulum disk link will open the actual model so you can try out the controls I describe here.

The general layout of this model emulates a piece of graph paper, without the lines on it. The mouse cursor position is shown in the bottom margin in Cartesian coordinates (x,y). The vertical (y) extent of the coordinates system is set by values entered in the limit boxes in the left margin. The limit boxes in the bottom margin set the horizontal extent of the coordinate system. The model view shown is of an object representing a rigid disk suspended from a pivot pin. In this model the aspect ratio is fixed at the normal computer screen 4 by 3, so that the object shape and angular relationships are preserved. Changing either vertical or horizontal coordinate system limits will automatically change the other limits to maintain the aspect ratio.

In the left margin of models there is a cluster of pan/zoom controls. Around the center of the cluster are buttons for shifting the view window up, down, left, or right. Each click on one of these buttons shifts the view window by 1/2 its extent. Changing the coordinate system limits does this shifting. In the center of the cluster are three buttons for zooming. The F (farther) button zooms out by expanding the coordinate system limits 25%. The N (nearer) button zooms in by shrinking the coordinate system limits by 20%. The differences in percent make the F and N buttons undo one another's action. The B (back) button restores the pre-set coordinate limits, restoring the original image size.

The colors displayed on the models are mainly taken from your computers color scheme. The margin color is taken from the background color of your desktop. The drawing area color is your window background color. The drawing area is outlined in the color of your system information background. The text color for text in the margins is your windows text color. The object and vector colors are set by the model drawing algorithms.

This is an animated model. The animation is driven by data stored in a memory array on model startup. There is a button labeled Action, to start the animation. A Cut button allows you to halt the animation at any point. If you let it run, the animation stops automatically when a pre-set time limit is reached. A Clear button is provided to reset the model to its pre-action condition or, if you choose, to recalculate the model time-series data. If for some reason you want to hide the vectors that make up the free-body diagram for the model, the Hide FBD button toggles the vector display off and on.