@online{analyse-box2d,
    author = "Erin Catto",
    title  = "Box2D : A 2D Physics Engine for Games",
    howpublished = "\url{http://goo.gl/uTnXH4}"
}

@online{ptf-collision-response,
    author = "Randy Gaul",
    title  = "How to Create a Custom 2D Physics Engine: The Basics and Impulse Resolution",
    howpublished = "\url{http://goo.gl/G0gdWU}"
}