This document is a high-level description of the model used in PhET's Normal Modes simulation.
The One Dimension screen offers a variety of tools to represent oscillatory normal modes in one dimension.
To adjust the masses' movement pattern, you can both:
- manually drag them on the screen (even while moving or paused!)
- adjust each normal mode's amplitude and phase
The movement axis can be chosen through arrow buttons, which represent horizontal and vertical orientations.
To adjust each normal mode individually, you can set their amplitude and phase values using sliders on Normal Mode Spectrum panel.
Phase controls can also be shown/hidden using the Show Phases checkbox in the side menu to get some more screen space.
Note that you can see live animated curve representations through the Normal Modes panel, which represent, in fact, each mode's motion pattern.
The Two Dimensions screen is similar to One Dimension screen, with only a few differences.
In this screen, only normal modes' amplitudes are individually adjustable, and the controls are also a bit different.
To adjust each normal mode amplitude (Normal Mode Amplitudes panel):
- select the desired axis using the arrow buttons
- choose a rectangle by their row and column, which represent the normal mode number
- click on a rectangle to turn given normal mode amplitude "on and off"
Note that the amplitude values are represented by a progress bar inside each rectangle, from the bottom to the top.
It's also possible to control more general simulation settings through control panel at the upper-right:
- pause, step and resume the simulation
- manage simulation speed
- change number of masses
- show/hide all the springs
To return to previous states, there are utility buttons:
- Initial Positions: pauses the sim and sets mass positions to where they were when the simulation started moving
- Zero Positions: resets mass positions to their default value