Physics-based drawing app

Physics-based drawing app

I'm working on a project that's rather complex for me, considering that I
only recently picked up javascript.
Basically I want to use the box2d phsyics engine and I want to allow users
to draw on on a canvas and then I want box2d physics to act on the objects
the users have drawn.
I worked through these two tutorials and then played around with the
results a little bit:
http://net.tutsplus.com/tutorials/html-css-techniques/build-your-first-game-with-html5/
http://www.williammalone.com/articles/create-html5-canvas-javascript-drawing-app/
So now I have a little drawing app as well as a physics based game. I have
no idea whatsoever how to combine this in any way.
I'd be very grateful if anyone had any sort of general pointers or advice
for me on how I would approach this.