Physics Sketchpad has one of the worst possible UIs.
Square - Draw a rectangular solid
Circle - Draw a circular solid
Circle with Cross - Draw a wheel
Three wavy lines - Draw a constraint
Green Circle - Play/Pause
X - Fix/UnFix
+ - Grab and move
U shape - Rotate
Arrow - Attribute menu (manually adjust attributes of an object)
Three horizontal lines - Open saved project menu

If I write a Physics Sketchpad v.2.0, I will redo the whole menu.  I even promise you tool tips for the shortcuts.