screen height can reduced a bit, currently can't see simulation n buttons in one view
The simulation is pretty cool, emphasis on the gradient concept of a X vs t graph gives the velocity.
I like it!

notice: t = 0 because when the field in [glow=yellow,2,300]yellow[/glow], user need to press enter key, so u need to inform them to remember to press enter? or has EJS 4.0 improved to take values in autodetect?

About the submit answer, can other people like me make assessment/ feedback strategies on our own simulation?

[b]For Info:[/b]
I was doing something like this before
My Old Website:
My EJS Simulation on S.H.M.
Example Quiz: [color=blue][b]Click on that! [/b][/color]
Very labor intensive for a good assessment and hints etc.
or a direct link