Operation

The interface is not very Mac like. The rationalization is that I want to keep it portable. The reason is that it is too much work.

Commands are keystrokes. They mostly come in matched pairs—upper and lower case letters are inverses. The keystrokes are buffered and cumulative so that you can type them without waiting for previous ones to take effect.

Gg

Starts(Go) and stops the computation so that you can inspect a static configuration. Time stops, momentum is remembered.

Arrow keys

The four keyboard arrows move the image. (Pan)

Ss

Expands and shrinks the image. (Zoom)

Rr

rotates the image to the right about a vertical axis in the screen.

Uu

rotates the image up (U) or down (u) about a horizontal axis in the screen.

Tt

turns the image about an axis perpendicular to the screen.

Mm

Magnifies or diminishes the amount of the above adjustments.

Vv

is supposed to turn on and off the plotting of velocities but it is broke.

q

returns to the application’s invoker.

Dd

turns damping on and off. It is initially off. Damping deducts 1% of momentum per cycle.

F

releases the constraints and sets the velocities to 0.

The shape in the demo is a bent rod 11 units long and with a heptagonal cross section. Until the command “F” is pressed the two ends are confined to two planes slowly approaching each other. The configuration is initially unstrained.

The following script may be followed for a guided tour: This is script for a screen of 1152 by 870 pixels. It runs several times as fast if your monitor is set to two colors (black and white). This may differ for machines with decent graphics cards. The app is not a god Mac screen citizen. It hogs the machine until you type “q”, and then it does not cause the screen’s original pixels to be replaced.

1. Launch the app. (It works on Mac OS versions 7 thru 8.6)
2. Type 8 “s” to shrink the image to fit better on the screen.
3. Type about 18 up arrows and 16 right arrows to center the figure.
4. Type 16 “R” and 18 more “s” for a better vantage point.
5. Type 6 right arrows.
6. Type 12 “u” and see the bend in the rod.
7. Turn on time by typing πG∏. Observe the counter in upper left of screen.
8. On a 60 MHz power PC it runs about 20 cycles per second.
9. At about cycle 300 turn time off with “g”.
10. Type “G” to resume time and “F” to liberate the rod.
11. Watch it writhe as mostly potential strain energy converts to kinetic energy.