I may be "knocking down an open door" but I just want to point out that many of the Physlet applets can be used even without Javascript code. in the applet on this page there is the
<param name="ShowControls" value="false">
If that is set to "true" instead, the page visitor will see a control panel and be able to create and change objects.

But the true power of Physlets is the ability to create situations that can be easily manipulated (by the visitor pressing a button or changing a value) to show the things one wants to show.