now that chem uses browserify, you can use one of these:
$ npm search box2d
NAME DESCRIPTION
box2d 2D physics engine
box2dnode 2D physics engine
box2dweb-commonjs Box2dweb as a Common JS module
box2dweb-multiplayer-demo a simple multiplayer Box2DWeb demo using node.js and s
pong-box2d Pong game engine for node and browser, depends on box2dweb
now that chem uses browserify, you can use one of these: