Being members of CoreDev means that other people will read the code we write. We should avoid embedding/exposing complicated functions from the underlying libraries (CP, gosu, etc) in game-side code (like the grenade) whenever possible.

That being said, moment_for_circle creates the moment of inertia for a hollow circle (ie a ring). Grenades are not hollow circles. The only other options chipmunk provides are moments for line segments, polygons and boxes. Since this is only for rotation, and since everything is sprites anyway, we might as well treat things as boxes when calculating the moment of inertia. My suggestion is thus to automatically generate the moment in Dynen, based on the w() and h() of the entity.